Lexeme (i.e. lexical item)

The term lexeme is sometimes used in the same sense as lexical item, i.e. an expression that is not transparently derivable from more elementary expressions.

Examples
In addition to words (in particular lexemes in the sense of sets of word-forms), idioms such as kick the bucket are also lexemes in this sense.

Polysemy

 * lexeme (i.e. set of word-forms)

Origin
The term was first used by Louis Hjelmslev and Benjamin L. Whorf in the 1930s. Apparently both coined the term independently.