Passive vocabulary refers to the words that a person understands but does not actively use in speech or writing. This vocabulary is often recognized in reading or listening contexts, where individuals can comprehend the meaning of words without necessarily incorporating them into their own communication. For example, when someone reads a book or listens to a conversation, they may encounter many words that they know but rarely say themselves. This is common in learning languages, where learners build a larger passive vocabulary before developing their active vocabulary for speaking and writing.