A morpheme is the smallest meaningful lexical item in a language. The field of linguistic study dedicated to morphemes is called morphology.

In English, morphemes are often, but not necessarily, words. Those that stand alone are considered roots (such as the morpheme cat); other morphemes, called affixes, are only found in combination with other morphemes. For example the -s in cats indicates the concept of 'plurality' but is always bound to another concept to indicate a specific kind of plurality).[1]

This distinction is not universal and does not apply to, for example, Latin, where many roots cannot stand alone. For instance, the Latin root reg- (‘king’) must always be suffixed with a case marker: rex (reg-s), reg-is, reg-i, etc. For a language like Latin, a root can be defined as the main lexical morpheme of a word.

Bound morphemes can be further classified as derivational or inflectional morphemes. The main difference between them is their function in relation to words.

Allomorphs are variants of a morpheme that differ in form but are semantically similar. For example, the English plural marker has three allomorphs: /-z/ (bugs), /-s/ (bats), or /-ɪz, -əz/ (buses). An allomorph is a concrete realization of a morpheme, which is an abstract unit. This is parallel to the relation of an allophone and a phoneme.

A zero-morpheme is a type of morpheme that carries semantic meaning but is not represented by auditory phonemes. They are often represented by /Ø/ within glosses.[9]

Generally, these types of morphemes have no visible changes. For instance, sheep is both the singular and the plural form. The intended meaning is thus derived from the Co-occurrence determiner (in this case, "some-" or "a-").[10]

Content morphemes express a concrete meaning or content, and function morphemes have more of a grammatical role. For example, the morphemes fast and sad can be considered content morphemes. On the other hand, the suffix -ed is a function morpheme since it has the grammatical function of indicating past tense.

Both categories may seem very clear and intuitive, but the idea behind them is occasionally harder to grasp since they overlap with each other.[11] Examples of ambiguous situations are the preposition over and the determiner your, which seem to have concrete meanings but are considered function morphemes since their role is to connect ideas grammatically.[12] Here is a general rule to determine the category of a morpheme:

Roots are composed of only one morpheme, while stems can be composed of more than one morpheme. Any additional affixes are considered morphemes. For example, in the word quirkiness, the root is quirk, but the stem is quirky, which has two morphemes.

Moreover, some pairs of affixes have identical phonological form but different meanings. For example, the suffix -er can be either derivational (e.g. sellseller) or inflectional (e.g. smallsmaller). Such morphemes are called homophonous.[12]

Some words might seem to be composed of multiple morphemes but are not. Therefore, not only form but also meaning must be considered when identifying morphemes. For example, the word relate might seem to be composed of two morphemes, re- (prefix) and the word late, but it is not.[citation needed] Those morphemes have no relationship with the definitions relevant to the word like "to feel sympathy," "to narrate," or "to be connected by blood or marriage." By contrast, the word rename does consist of two morphemes; here the morpheme re- indicates that the action "name" is done again.

Furthermore, the length of a word does not determine whether or not it has multiple morphemes. The word Madagascar is long and might seem to have morphemes like mad, gas, and car, but it does not. Conversely, some short words have multiple morphemes (e.g. dogs = dog + s).[12]

Morphological icons are images, patterns or symbols that relate to a specific morpheme.[13] For children with dyslexia, it has been shown to be an effective way of building up a word. The word 'inviting' as an example is made up of two commonly used morphemes, 'in-' and '-ing'. A morphological icon for 'in-' could be an arrow going into a cup, and '-ing' could be an arrow going forward to symbolise that something is in action (as in being, running, fishing).

The concept of combining visual aid icons with morpheme teaching methods was pioneered from the mid 1980s by Neville Brown.[14] He founded the Maple Hayes school for dyslexia in 1981, where he later improved the method alongside his son, Daryl Brown. The school's curriculum uses morphological icons as a learning aid.[15]

In natural language processing for Japanese, Chinese, and other languages, morphological analysis is the process of segmenting a sentence into a row of morphemes. Morphological analysis is closely related to part-of-speech tagging, but word segmentation is required for these languages because word boundaries are not indicated by blank spaces.[citation needed]

The purpose of morphological analysis is to determine the minimal units of meaning in a language (morphemes) by comparison of similar forms: for example, comparing forms such as "She is walking" and "They are walking" with each other rather than either with something less similar like "You are reading." The forms can be effectively broken down into parts and the different morphemes can be distinguished.

Both meaning and form are equally important for the identification of morphemes. An agent morpheme is an affix like -er that in English transforms a verb into a noun (e.g. teachteacher). English also has another morpheme that is identical in pronunciation (and written form) but has an unrelated meaning and function: a comparative morpheme that changes an adjective into another degree of comparison (but remains the same adjective) (e.g. smallsmaller). The opposite can also occur: a pair of morphemes with identical meaning but different forms.[12]

In generative grammar, the definition of a morpheme depends heavily on whether syntactic trees have morphemes as leaves or features as leaves.

Given the definition of a morpheme as "the smallest meaningful unit," nanosyntax aims to account for idioms in which an entire syntactic tree often contributes "the smallest meaningful unit." An example idiom is "Don't let the cat out of the bag." Here, the idiom is composed of "let the cat out of the bag." This might be considered a semantic morpheme that is itself composed of many syntactic morphemes. Other cases of the "smallest meaningful unit" being longer than a word include some collocations such as "in view of" and "business intelligence", in which the words together have a specific meaning.

The definition of morphemes also plays a significant role in the interfaces of generative grammar in the following theoretical constructs: