附着词素(clitic)在语言学上为词语在句法中之作用有如一自由词素，不过在语音学上却表现出规范词素的特性；附着词素的发音是跟随前一个字，或者后一个字来进行。附着词素之组成架构一则为前属词(enclitic)，在此附着词素是附着前一字词之后；二则为连接发音词(proclitic)，在此附着词素却是再接下一字词。某些语言中，比如：葡萄牙语中也存有附着词素类的中间词(mesoclitic)，就是将"中间附着词素"置放于字词中间。逐渐地，可以了解到许多附着词素之词语正在进行着文法化(grammaticalization)的演进程序：

In morphology and syntax, a clitic (/ˈklɪtᵻk/ 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.