Passive vocabulary refers to the words that a person understands but does not actively use in their speech or writing. This vocabulary is often recognized when reading or listening, allowing individuals to comprehend messages without necessarily being able to recall or use those words themselves. For example, a person may understand complex terms related to science or literature when reading but may not use them in everyday conversation. Passive vocabulary is essential for language development, as it helps individuals grasp the meaning of texts and engage with various subjects more effectively.