Retention is remembering new language rather than forgetting it. Once a word is retained, it can be retrieved and used later.
If learners have to work to guess or infer the meaning of a new word, and do so themselves, then retention of that word is more likely.
In the classroom
Activities which can help retention of new language include learners deducing meaning from context, recording tools such as mind maps and visualisation, using material relevant to the learners, and a range of approaches to suit learner styles.