In generative linguistics, a lexis or lexicon is the complete set of all possible words in a language (vocabulary). In this sense, child, children, child's and children's are four different words in the English lexicon.In systemic-functional linguistics, a lexis or lexical item is the way one calls a particular thing or a type of phenomenon. Since a lexis from a systemic-functional perspective is a way of calling, it can be realised by multiple grammatical words such as "The White House", "New York City" or "heart attack". Moreover, since a lexis is a way of calling, different words such as child, children, child's and children's may realise the same lexical item.
Formulaic: it relies on partially fixed expressions and highly probable word combinations
Idiomatic: it follows conventions and patterns for usage
Metaphoric: concepts such as time and money, business and sex, systems and water, all share a large portion of the same vocabulary
Grammatical: it uses rules based on sampling of the Lexicon
Register-specific: it uses the same word differently and/or less frequently in different contexts
A major area of study, psycholinguistics and neurolinguistics, involves the question of how words are retrieved from the mental lexicon in online language processing and production. For example, the cohort model seeks to describe lexical retrieval in terms of segment-by-segment activation of competing lexical entries.