A vocabulary is a set of familiar words within a person's language. A vocabulary, usually developed with age, serves as a useful and fundamental tool for communication and acquiring knowledge. Acquiring an extensive vocabulary is one of the largest challenges in learning a second language.
Vocabulary is commonly defined as "all the words known and used by a particular person". Knowing a word, however, is not as simple as merely being able to recognize or use it. There are several aspects of word knowledge that are used to measure word knowledge.
The first major change distinction that must be made when evaluating word knowledge is whether the knowledge is productive (also called achieve) or receptive (also called receive); even within those opposing categories, there is often no clear distinction. Words that are generally understood when heard or read or seen constitute a person's receptive vocabulary. These words may range from well-known to barely known (see degree of knowledge below). A person's receptive vocabulary is usually the larger of the two. For example, although a young child may not yet be able to speak, write, or sign, he or she may be able to follow simple commands and appear to understand a good portion of the language to which they are exposed. In this case, the child's receptive vocabulary is likely tens, if not hundreds of words, but his or her active vocabulary is zero. When that child learns to speak or sign, however, the child's active vocabulary begins to increase. It is also possible for the productive vocabulary to be larger than the receptive vocabulary, for example in a second-language learner who has learned words through study rather than exposure, and can produce them, but has difficulty recognizing them in conversation.
Productive vocabulary, therefore, generally refers to words that can be produced within an appropriate context and match the intended meaning of the speaker or signer. As with receptive vocabulary, however, there are many degrees at which a particular word may be considered part of an active vocabulary. Knowing how to pronounce, sign, or write a word does not necessarily mean that the word that has been used correctly or accurately reflects the intended message; but it does reflect a minimal amount of productive knowledge.
Within the receptive–productive distinction lies a range of abilities that are often referred to as degree of knowledge. This simply indicates that a word gradually enters a person's vocabulary over a period of time as more aspects of word knowledge are learnt. Roughly, these stages could be described as:
The differing degrees of word knowledge imply a greater depth of knowledge, but the process is more complex than that. There are many facets to knowing a word, some of which are not hierarchical so their acquisition does not necessarily follow a linear progression suggested by degree of knowledge. Several frameworks of word knowledge have been proposed to better operationalise this concept. One such framework includes nine facets:
Words can be defined in various ways, and estimates of vocabulary size differ depending on the definition used. The most common definition is that of a lemma (the uninflected or dictionary form; this includes walk, but not walks, walked or walking). Most of the time lemmas do not include proper nouns (names of people, places, companies, etc). Another definition often used in research of vocabulary size is that of word family. These are all the words that can be derived from a ground word (e.g., the words effortless, effortlessly, effortful, effortfully are all part of the word family effort). Estimates of vocabulary size range from as high as 200 thousand to as low as 10 thousand, depending on the definition used. 
A literate person's vocabulary is all the words they can recognize when reading. This is generally the largest type of vocabulary simply because a reader tends to be exposed to more words by reading than by listening.
A person's listening vocabulary is all the words they can recognize when listening to speech. People may still understand words they were not exposed to before using cues such as tone, gestures, the topic of discussion and the social context of the conversation.
A person's speaking vocabulary is all the words they use in speech. It is likely to be a subset of the listening vocabulary. Due to the spontaneous nature of speech, words are often misused. This misuse, though slight and unintentional, may be compensated by facial expressions and tone of voice.
Words are used in various forms of writing from formal essays to social media feeds. Many written words do not commonly appear in speech. Writers generally use a limited set of words when communicating. For example, if there are a number of synonyms, a writer may have a preference as to which of them to use, and they are unlikely to use technical vocabulary relating to a subject in which they have no knowledge or interest.
The American philosopher Richard Rorty characterized a person's "final vocabulary" as follows:
All human beings carry about a set of words which they employ to justify their actions, their beliefs, and their lives. These are the words in which we formulate praise of our friends and contempt for our enemies, our long-term projects, our deepest self-doubts and our highest hopes… I shall call these words a person's “final vocabulary”. Those words are as far as he can go with language; beyond them is only helpless passivity or a resort to force. (Contingency, Irony, and Solidarity p. 73)
Focal vocabulary is a specialized set of terms and distinctions that is particularly important to a certain group: those with a particular focus of experience or activity. A lexicon, or vocabulary, is a language's dictionary: its set of names for things, events, and ideas. Some linguists believe that lexicon influences people's perception of things, the Sapir–Whorf hypothesis. For example, the Nuer of Sudan have an elaborate vocabulary to describe cattle. The Nuer have dozens of names for cattle because of the cattle's particular histories, economies, and environments[clarification needed]. This kind of comparison has elicited some linguistic controversy, as with the number of "Eskimo words for snow". English speakers with relevant specialised knowledge can also display elaborate and precise vocabularies for snow and cattle when the need arises.
During its infancy, a child instinctively builds a vocabulary. Infants imitate words that they hear and then associate those words with objects and actions. This is the listening vocabulary. The speaking vocabulary follows, as a child's thoughts become more reliant on his/her ability to self-express without relying on gestures or babbling. Once the reading and writing vocabularies start to develop, through questions and education, the child starts to discover the anomalies and irregularities of language.
In first grade, a child who can read learns about twice as many words as one who cannot. Generally, this gap does not narrow later. This results in a wide range of vocabulary by age five or six, when an English-speaking child will have learned about 1500 words.
Vocabulary grows throughout our entire life. Between the ages of 20 and 60, people learn some 6,000 more lemmas, or one every other day. An average 20-year-old knows 42,000 words coming from 11,100 word families; an average 60-year-old knows 48,200 lemmas coming from 13,400 word families. People expand their vocabularies by e.g. reading, playing word games, and participating in vocabulary-related programs. Exposure to traditional print media teaches correct spelling and vocabulary, while exposure to text messaging leads to more relaxed word acceptability constraints.
Estimating average vocabulary size poses various difficulties and limitations due to the different definitions and methods employed such as what is the word, what is to know a word, what sample dictionaries were used, how tests were conducted, and so on. Native speakers' vocabularies also vary widely within a language, and are dependent on the level of the speaker's education.
One most recent 2016 study shows that 20-year-old English native speakers recognize on average 42,000 lemmas, ranging from 27,100 for the lowest 5% of the population to 51,700 lemmas for the highest 5%. These lemmas come from 6,100 word families in the lowest 5% of the population and 14,900 word families in the highest 5%. 60-year-olds know on average 6,000 lemmas more. 
According to another, earlier 1995 study junior-high students would be able to recognize the meanings of about 10,000–12,000 words, whereas for college students this number grows up to about 12,000–17,000 and for elderly adults up to about 17,000 or more.
For native speakers of German average absolute vocabulary sizes range from 5,900 lemmas in first grade to 73,000 for adults.
The knowledge of the 3000 most frequent English word families or the 5000 most frequent words provides 95% vocabulary coverage of spoken discourse. For minimal reading comprehension a threshold of 3,000 word families (5,000 lexical items) was suggested and for reading for pleasure 5,000 word families (8,000 lexical items) are required. An "optimal" threshold of 8,000 word families yields the coverage of 98% (including proper nouns).
Learning vocabulary is one of the first steps in learning a second language, but a learner never finishes vocabulary acquisition. Whether in one's native language or a second language, the acquisition of new vocabulary is an ongoing process. There are many techniques that help one acquire new vocabulary.
Although memorization can be seen as tedious or boring, associating one word in the native language with the corresponding word in the second language until memorized is considered one of the best methods of vocabulary acquisition. By the time students reach adulthood, they generally have gathered a number of personalized memorization methods. Although many argue that memorization does not typically require the complex cognitive processing that increases retention (Sagarra and Alba, 2006), it does typically require a large amount of repetition, and spaced repetition with flashcards is an established method for memorization, particularly used for vocabulary acquisition in computer-assisted language learning. Other methods typically require more time and longer to recall.
Some words cannot be easily linked through association or other methods. When a word in the second language is phonologically or visually similar to a word in the native language, one often assumes they also share similar meanings. Though this is frequently the case, it is not always true. When faced with a false friend, memorization and repetition are the keys to mastery. If a second language learner relies solely on word associations to learn new vocabulary, that person will have a very difficult time mastering false friends. When large amounts of vocabulary must be acquired in a limited amount of time, when the learner needs to recall information quickly, when words represent abstract concepts or are difficult to picture in a mental image, or when discriminating between false friends, rote memorization is the method to use. A neural network model of novel word learning across orthographies, accounting for L1-specific memorization abilities of L2-learners has recently been introduced (Hadzibeganovic and Cannas, 2009).
One useful method of building vocabulary in a second language is the keyword method. If time is available or one wants to emphasize a few key words, one can create mnemonic devices or word associations. Although these strategies tend to take longer to implement and may take longer in recollection, they create new or unusual connections that can increase retention. The keyword method requires deeper cognitive processing, thus increasing the likelihood of retention (Sagarra and Alba, 2006). This method uses fits within Paivio's (1986) dual coding theory because it uses both verbal and image memory systems. However, this method is best for words that represent concrete and imageable things. Abstract concepts or words that do not bring a distinct image to mind are difficult to associate. In addition, studies have shown that associative vocabulary learning is more successful with younger students (Sagarra and Alba, 2006). Older students tend to rely less on creating word associations to remember vocabulary.
Several word lists have been developed to provide people with a limited vocabulary either for the purpose of rapid language proficiency or for effective communication. These include Basic English (850 words), Special English (1,500 words), General Service List (2,000 words), and Academic Word List. Some learner's dictionaries have developed defining vocabularies which contain only most common and basic words. As a result word definitions in such dictionaries can be understood even by learners with a limited vocabulary. Some publishers produce dictionaries based on word frequency or thematic groups.