"Spaced repetition is a learning technique that incorporates increasing intervals of time between subsequent review of previously learned material in order to exploit the psychological spacing effect."
"In the Leitner system, correctly answered cards are advanced to the next, less frequent box, while incorrectly answered cards return to the first box for more aggressive review and repetition."
"Active recall is a principle of efficient learning, which claims the need to actively stimulate memory during the learning process. It contrasts with passive review, in which the learning material is processed passively (e.g. by reading, watching, etc.)."