Lexis – this linguistic category falls under word construction and meaning and generally defined as the total word stock and usage that depends on choice and appropriateness. The presence of multi-word lexical items in the lexis is what differentiates it from vocabulary - the collection of only single words.
traffic light, take care of, by the way and don't count your chickens before they hatch.
Lexis means the vocabulary of a language as opposed to other aspects such as the grammar of the text. Lexis is clearly an important aspect of creating a suitable style or register (i.e. when choosing language and language features to suit a particular genre, context, audience and purpose).
Lexis and semantics are very close and often used interchangeably.
Lexical cohesion occurs when words have an affinity for each other as in collocations.Many words are habitually put together - or collocated. A collocation is any habitually linked group of words - a kind of lexical partnership, e.g. 'fish and chips', 'salt and pepper', 'don't mention it', 'it's nothing...', 'Oh well!', 'bangers and mash'... and so on.