Teachers have always known that a very significant part of learning a subject is learning its vocabulary. All students expect to learn subject-specific terms such as evaporation, denominator, reafforestation, etc. These words are generally not such a problem for ESL students because they are usually the focus of the discussion in the classroom or are glossed in the textbook.
In fact, the words that ESL students have most difficulty with are the academic, non-subject-specific words* that occur across all disciplines; words like duration, indicate, similarity, furthermore, rate, etc. These words cause ESL students problems for two reasons:
subject teachers generally assume that their students know these words and seldom teach them explicitly;
most of the words are abstract, which makes them much more difficult to learn.
Implications for mainstream teachers:
It is helpful simply to have an awareness of the problem stated above. This may explain why students are having difficulty comprehending a text about the water cycle, for example, when you know that they have fully understood the concepts of evaporation, condensation etc. It would be useful from time to time to make explicit the meanings of some of the semi-technical vocabulary that seems to reoccur in your subject. In fact, doing so will not only help the ESL students in your classes, but will also be of benefit to many native-speakers too, who have only a hazy understanding of the terms.
Without a strong academic vocabulary ESL students will struggle to achieve success in your class.