Samhita literally means "put together, joined, union", a "collection", and "a methodically, rule-based combination of text or verses". Samhita also refers to the most ancient layer of text in the Vedas, consisting of mantras, hymns, prayers, litanies and benedictions.
Samhita is a Sanskrit word from the prefix sam (सम्), 'together', and hita (हित), the past participle of the verbal root dha (धा) 'put'.  The combination word thus means "put together, joined, compose, arrangement, place together, union", something that agrees or conforms to a principle such as dharma or in accordance with justice, and "connected with". Saṃhitā (संहिता) in the feminine form of the past participle, is used as a noun meaning "conjunction, connection, union", "combination of letters according to euphonic rules", or "any methodically arranged collection of texts or verses".
Samhita, however, in contemporary literature typically implies the earliest, archaic part of the Vedas. These contain mantras – sacred sounds with or without literal meaning, as well as panegyrics, prayers, litanies and benedictions petitioning nature or Vedic deities. Vedic Samhita refer to mathematically precise metrical archaic text of each of the Vedas (Rigveda, Yajurveda, Samaveda and Atharvaveda).
The Vedas have been divided into four styles of texts – the Samhitas (mantras and benedictions), the Brahmanas(text on rituals, ceremonies, sacrifices and symbolic-sacrifices) , the Aranyakas (commentaries on rituals, ceremonies and sacrifices), and the Upanishads (text discussing meditation, philosophy and spiritual knowledge). The Samhitas are sometimes identified as karma-khanda (कर्म खण्ड, action/ritual-related section), while the Upanishads are identified as jnana-khanda (ज्ञान खण्ड, knowledge/spirituality-related section). The Aranyakas and Brahmanas are variously classified, sometimes as the ceremonial karma-khanda, other times (or parts of them) as the jnana-khanda.
A collective study of Vedas and later text suggests that the compendium of Samhitas and associated Vedic texts were far larger than currently available. However, most have been lost at some point or over a period of Indian history.
Weber noted that the Samhita of Samaveda is an anthology taken from the Rigveda-Samhita. The difference is in the refinement and application of arts such as melody, meters of music and literary composition. Thus, the root hymn that later became the Rathantara (Excellent Chariot) mantra chant is found in both Rigveda and Samaveda Samhitas, as follows,
May the Vasus prepare you, with the gayatri meter, you are the earth,
May the Rudras prepare you, with the tristubh meter, you are the sky.
May the Adityas prepare you, with the jagati meter, you are the heaven.
May the Visvedevas, common to all men, prepare you, with the anustubh meter, you are the directions.
You are the unchanging direction, make unchanging in me children, abundance of wealth, abundance of cattle, abundance of heroism.
May O Agni!, a suitor after this girl's heart come to her,
May he come to this maiden with fortune!
May she be agreeable to suitors, charming at festivals, promptly obtain happiness through a husband!
There are many well known books written in the post-vedic period, also known as samhitas, because the word “samhita” also means “systematic compilation of knowledge”. Vedic samhitas should not be confused with these samhitas of post-vedic period.
Some post-vedic Samhitas are: Ashtavakra Gita, Bhrigu Samhita, Brahma Samhita, Charaka Samhita, Deva Samhita, Garga Samhita, Gheranda Samhita, Kashyap Samhita, Shiva Samhita, Sushruta Samhita (a treatise on food and medicine), Yogayajnavalkya Samhita.