A learner's passive vocabulary is the words that they understand but don't use yet. This can be compared with active vocabulary, which are words that learners understand and use in speaking or writing. The active and passive vocabulary of a learner changes constantly. They start using words, try new meanings, forget words, abandon words that have no use, revise words, etc.
Advanced learners often have an extremely large passive vocabulary but a considerably smaller active one.
In the classroom
A simple way to help learners keep building their active vocabulary is by constant revision activity, such as asking learners to use words left on the board from a previous class in a sentence. Learners can all contribute to a vocabulary bag with words and definitions or example sentences on cards; these can be used to play vocabulary revision games.