在语素构词学里，语素(Morpheme)（或翻译成「义基」）是最小的语法单位，是最小的语音语义结合体。

在口语形式语言中，语素是由「音位」这一种能区别的最小声音单位所组成的，而在文本形式语言中，语素则是由「字位」所组成的。因为汉语是一字一音，所以可按照音位的个数，区分成单音节语素、双音节语素、多音节语素和非音节语素。

语素和词不同，有许多的语素不能够单独地构成词。依据是否可单独成词，语素可分为自由语素和不自由语素。

In linguistics, a morpheme is the smallest grammatical unit in a language. In other words, it is the smallest meaningful unit of a language. The field of study dedicated to morphemes is called morphology. A morpheme is not identical to a word, and the principal difference between the two is that a morpheme may or may not stand alone, whereas a word, by definition, is freestanding. When it stands by itself, it is considered a root because it has a meaning of its own (e.g. the morpheme cat) and when it depends on another morpheme to express an idea, it is an affix because it has a grammatical function (e.g. the –s in cats to indicate that it is plural). Every word comprises one or more morphemes.