Expressive v. Receptive Vocabulary
A word is in your receptive vocabulary when you understand it when others use it. A word is in your expressive vocabulary when you think to use it. Well over half of the average adult's vocabulary is receptive.
To move a word from your receptive to your expressive vocabulary, periodically review it until it comes to mind habitually.
You may also encounter this concept under other labels such as active v. passive or productive v. receptive vocabulary.