clitic

In morphology and syntax, a clitic ( from Greek κλιτικός klitikos, "inflexional") is a morpheme that has syntactic characteristics of a word, but depends phonologically on another word or phrase. In this sense, it is syntactically independent but phonologically dependent, always attached to a host. The term is derived from the Greek for "leaning". It is pronounced like an affix, but plays a syntactic role at the phrase level. In other words, clitics have the form of affixes, but the distribution of function words. For example, the contracted forms of the auxiliary verbs in I'm and we've are clitics.