A word or group of words that refers to a single concept: table / timetable / lay the table hat / opera hat / eat your hat.
Any item which functions as a single unit in the lexicon. ÔFly' is a lexical item. ÔOn the fly' is also a lexical item, though it consists of three words. The lemma "Watch", `watching', `watches', and Ôwatched' can be represented as one lexical item WATCH.
An item of vocabulary which has a single element of meaning. It may be a compound or phrase: bookcase, post office, put up with. Some single words may initiate several lexical items; eg letter: a letter of the alphabet / posting a letter.