Sometimes the basic unit is the phrase, but some other times the basic unit is the phoneme or if one's studying language as a system of writing it might be the syllable.

有时候基本单位是一个词组，有时候则是一个音素,也就是在写作的研究中的,一个音节。

耶鲁公开课 - 文学理论导论课程节选