clitic

Merriam-Webster's Word of the Day for September 03, 2010 is:

clitic • \KLIT-ik\  • noun
: a word that is treated in pronunciation as forming a part of a neighboring word and that is often unaccented or contracted

Example sentence:
In what's happening? the s in what's is a clitic.

Did you know?
We hear clitics every day in sentences like This'll be fine and C'mon over here. There are two kinds of clitics: enclitics and proclitics. An enclitic is a clitic that is associated with the word that comes before it. Contractions, such as the ve in would've and the ll in it'll, are enclitics. A proclitic is associated with the word that follows it. Proclitics are transcribed into print far less often than enclitics are, but we hear them frequently in speech. For example, the sentence They love to dance is typically pronounced with the to truncated to a t that gets tacked onto the front of dance.