Retention refers to the ability to recall or reproduce learned knowledge components during a future knowledge event.
For example, if a student learns a word's pronunciation during training, retention would be demonstrated if the student can recall/reproduce that word's pronunciation at a later time. This can be distinguished from transfer, in which the student would apply the learned pronunciation to new words or new contexts.
Retention should not be confused with long-term retention. A retention test is a kind of normal post-test that occurs immediately after instruction and typically involves items much like those in the instruction.