A clitic (from Ancient Greek klitikón) is a word which bears no stress. By phonosyntactic rule, it forms a phonological unit with an adjacent word, called its host, whose stress becomes the stress of this phonological unit. Clitics are chiefly classified by their position relative to their host:

• An enclitic is a clitic following its host.
• A proclitic is a clitic preceding its host.

The specialized literature also mentions mesoclitics and endoclitics (analogous to interfixes and infixes, resp.).

A clitic may select a host of a special syntactic category. For instance, the Spanish clitic pronouns like me, te, le only combine with verb forms. Alternatively, a clitic may be positioned with respect to a syntactic boundary. For instance, the Hittite clitic pronouns occupy the second position in the clause (Wackernagel's position), no matter what occupies the first position.

As a consequence, the phonological host of a clitic does not necessarily coincide with its syntactic coconstituent. In particular, if the syntactic position of a clitic is on the right side of a constituent boundary, but it is phonologically enclitic, it attaches to whatever precedes it. This is true for the enclitic pronouns of Yucatec Maya (in, a ... ‘I, you’ ...); these precede their syntactic head, but combine phonologically with whatever precedes them.

The concept of the clitic is related to neighboring concepts as follows:

• A clitic may or may not be a particle. German halt Reaffirmed is a particle; German es ‘it’ is not.
• A clitic may or may not consist of a morpheme. German halt and es are morphemes; Ancient Greek tis ‘somebody’ is a word consisting of two morphemes, ti- ‘any’ and -s Nom.Sg.M.
• A clitic differs from an affix in several respects:
  • An affix is a morpheme; a clitic is a word.
  • While a clitic cannot be stressed, an affix may be stressed. For instance, the personal endings of the Spanish future conjugation (cantaré, cantarás etc. ‘I will sing, you will sing ...’) are stressed.
  • An affix occupies a fixed syntagmatic slot; it is either a prefix or a suffix or one of the other kinds of affix. A clitic may be proclitic in one context, enclitic in another (and possibly even mesoclitic in yet another).
  • An affix is category-sensitive; i.e. it combines with words of a particular word class or, if a phrasal affix, with phrases of a particular syntactic category. Some clitics do so, too. However, a clitic may also attach phonologically to whatever is on the yonder side of a syntactic boundary.