中介语（Interlanguage）是指从母语(L1)始至习得第二语言(L2)止，其间经历的一系列的语言过渡「阶段」里，所产生之「语言」。换言之，中介语认为学习者犯的错不但有可预测性，更能依能力高低划出犯错的不同阶段，这些「错误」的语言似乎也有自己独立的体系。研究这套逻辑，累积犯错的模式，便能帮助第二语言习得的教师预测学生可能会犯的错误，并加以强调、防范。另外，即使是高程度的学习者、居住在第二语言国家的非母语用户（Non-native speaker，NNS），甚至是非母语之外语教师（NNSEFL teacher）都会面对中介语的问题。

Interlanguage is the term for a dynamic, rule-based linguistic system that has been developed by a learner of a second language (or L2) who has not yet reached proficiency. A learner's interlanguage preserves some features of their first language (or L1), and can also overgeneralize some L2 writing and speaking rules. These two characteristics of an interlanguage result in the system's unique linguistic organization.