Clitic

In morphology and syntax, a clitic is a morpheme that has syntactic characteristics of a word, but depends phonologically on another word or phrase. It is pronounced like an affix, but works at the phrase level. For example, the word an in the phrase:

an apple

Clitics may belong to any grammatical category, though they are commonly pronouns, determiners or adpositions. Note that orthography is not a good guide for identifying clitics: clitics may be written as independent words, bound affixes, or separated by special characters (e.g., apostrophe).