lexeme

Definition:

The fundamental unit of the lexicon (or word stock) of a language. Some lexemes (such as put up with) consist of more than one word.

Etymology:

From the Greek, "word, speech"

Examples and Observations:

• "A lexeme is a unit of lexical meaning, which exists regardless of any inflectional endings it may have or the number of words it may contain. Thus, fibrillate, rain cats and dogs, and come in are all lexemes, as are elephant, jog, cholesterol, happiness, put up with, face the music, and hundreds of thousands of other meaningful items in English. The headwords in a dictionary are all lexemes."
  (David Crystal, The Cambridge Encyclopedia of the English Language, 2nd ed. Cambridge University Press, 2003)
• "In many cases it makes no difference whether we take a syntactic or a lexical perspective. Lexemes such as the and and are invariable, i.e., there is only one word corresponding to each. Also invariable are lexemes like efficiently: although more efficiently is in some respects like harder, it is not a single word, but a sequence of two, and hence efficiently and more efficiently are not forms of a single lexeme. Variable lexemes, by contrast, are those which have two or more forms. Where we need to make clear that we are considering an item as a lexeme, not a word, we will represent it in bold italics. Hard, for example, represents the lexeme which has hard and harder--and also hardest--as its forms. Similarly are and is, along with be, been, being, etc., are forms of the lexeme be. . . . A variable lexeme is thus a word-sized lexical item considered in abstraction from grammatical properties that vary depending on the syntactic construction in which it appears."
  (Rodney Huddleston and Geoffroy Pullum, The Cambridge Grammar of the English Language. Cambridge Univ. Press, 2002)

Pronunciation: LECK-seem

source: grammar.about.com