In the simplest case, a lexical unit is defined as a triple consisting of a lemma, a part of speech, and a frame: a lemma in turn is a set of word form alternatives, and is named by one of its variants (the so-called "citation form").
the lemma ring as a noun has word-forms ring and rings (singular and plural); the lemma ring as a verb has word-forms ring, rings, ringing, rang, rung; the noun lemma ring when paired with a SOCIAL COMMUNITY frame (as in ring of smugglers) is one lexical unit; when paired with a JEWELRY frame (as in wedding ring) is a different lexical unit.