Retention of knowledge over a relatively long period of time. In PSLC studies, a post-test given immediately after the instruction is not a long-term retention test. As a weak heuristic, a long-term retention interval (the time between the end of instruction and the test) should be as least as long as the time of the instruction (the time between the beginning and end of the instructional period of the study). Difference between control and treatment instruction tend to be harder to detect at longer retention intervals but the longer the interval at which a difference is detected the greater the evidence of the treatment leading to long-term retention.
In one of the theories of learning, there are two parts to developing long-term retention: memorization (the transfer of new knowledge from the short term memory to the long term memory) and incorporation (relating new knowledge to exisiting knowledge and assigning it to a proper location in your long term memory ).