Academic language is the language used in textbooks, in classrooms, and on tests. It is different in structure and vocabulary from the everyday spoken English of social interactions. Many students who speak English well have trouble comprehending the academic language of high school and college classrooms. Low academic language skills have been shown to be associated with low academic performance in a variety of educational settings. The main barrier to student comprehension of texts and lectures is low academic vocabulary knowledge.
Academic vocabulary is sub-technical vocabulary.
In other words, it is not the technical vocabulary of a particular academic discipline. Academic vocabulary is used across all academic disciplines to teach about the content of the discipline. For example, before taking chemistry, no students know the technical words used in chemistry. But the underprepared students also don't know the vocabulary used to teach the chemistry concepts. Underprepared students are unfamiliar with words like evaluation, theory, hypothesis, assumption, capacity, validate.