Lexicology (from Greek lexis «word» and logos «learning») is a branch of Linguistics studying the vocabulary of the language. The word is the subject matter of Lexicology. All the words in a language make up its vocabulary. The term «vocabulary» is used to denote «the system formed by the sum total of all the words that the language posesses» (I.V.Arnold). Distinction is made between General Lexicology and Special Lexicology; Historical Lexicology, or Etymology, and Descriptive Lexicology according to the chosen approach: diachronic or synchronic approach. There is also Comparative Lexicology and Contrastive Lexicology.
“The word is the basic unit of the language resulting from the association of a particular meaning with a particular group of sounds, capable of a particular grammatical employment» (I.V.Arnold). It is the smallest significant unit of a given language, that can be functioning alone and characterized by positional mobility within a sentence, morphological uninterruptability and semantic integrity.