Spaced repetition glossary

Plain definitions of the terms used in spaced repetition, memory research and flashcard apps such as Anki. Most terms link to the article that explains them in detail.

Active recall

Studying by retrieving information from memory, for example answering a question before checking the answer, instead of rereading it.

Researchers usually call it retrieval practice.

Card

A question and answer pair that you review, like a paper flashcard. Spaced repetition apps schedule each card separately.

In Anki, one note can produce several cards, for example one in each direction for a vocabulary word.

Cloze deletion

A card made by hiding part of a sentence, which you then have to fill in from memory.

Cramming

Studying something in one long session shortly before you need it, instead of spreading the same study time out.

Researchers call it massed practice. It can help on a test the next day, but the material is forgotten faster.

Desirable difficulty

A way of studying that feels harder at the time but helps you remember for longer, such as spacing out sessions or testing yourself.

The psychologist Robert Bjork coined the term in 1994.

Desired retention

In FSRS, the probability of recall you want a card to have when it comes up for review. Anki sets it to 90% by default.

A higher target gives shorter intervals and more reviews; a lower one gives longer intervals and more forgotten cards.

Read more: The FSRS algorithm

Difficulty

In FSRS, a value for how hard a card is for you. The harder a card, the less its stability grows after each successful review.

Read more: The FSRS algorithm

Due

A card is due when the date its scheduler set for the next review has arrived.

Ease factor

In SM-2, the number the previous interval is multiplied by to get the next one. It starts at 2.5, never goes below 1.3, and changes with how well you answer.

Anki calls it ease and shows it as a percentage, starting at 250%.

Ease hell

A term used by Anki users for cards whose ease has dropped so low after repeated failures that they keep coming back at short intervals.

Forgetting curve

A curve showing how memory of new information declines over time when it is not reviewed. Hermann Ebbinghaus first measured it in 1885.

FSRS

The Free Spaced Repetition Scheduler, an open-source algorithm that estimates how likely you are to recall each card and schedules a review when that probability falls to your desired retention.

It is available in Anki since version 23.10.

Read more: The FSRS algorithm

Interleaving

Mixing different types of problems or topics in one study session, instead of practising one type at a time (blocked practice).

Interval

The time between two reviews of the same card. In spaced repetition, intervals grow after each successful review.

Lapse

A failed review of a card you had already learned. In Anki, a lapse sends the card back to relearning and adds to its lapse count.

Learning steps

In Anki, the short intervals, in minutes or hours, that a new card goes through before it moves to intervals counted in days. The default steps are 1 minute and 10 minutes.

Leech

A card you keep forgetting, which costs far more review time than others. By default, Anki flags a card as a leech and suspends it after 8 lapses.

Rewriting the card is usually more effective than reviewing it again.

Leitner system

A way of scheduling paper flashcards with boxes: a card you answer correctly moves to a box reviewed less often, and a card you miss goes back to the first box.

Sebastian Leitner described it in 1972.

Memory palace

A memory technique in which you picture a familiar place and leave an image of each item to remember along a route through it. It is also called the method of loci.

Mnemonic

A memory aid that links information to something easier to remember, such as a word, a rhyme or an image.

Retention

How much of what you learned you still remember after some time. In flashcard apps, it usually means the share of reviews you answer correctly.

Retrievability

The probability that you can recall something right now. It falls as time passes since your last review.

Retrievability and stability come from Piotr Woźniak’s two-component model of memory, which SuperMemo and FSRS build on.

Read more: The FSRS algorithm

Retrieval practice

The research term for learning by recalling information from memory, for example with practice questions.

Review

One instance of seeing a due card, trying to recall the answer and rating how well you did. The rating decides when the card comes back.

Savings

The reduction in time or repetitions needed to relearn something, compared with learning it the first time. Ebbinghaus used it to measure forgetting.

Savings are not the share of material remembered: 34% savings after a day does not mean 66% was forgotten.

SM-2

The scheduling algorithm Piotr Woźniak wrote for SuperMemo in 1987. Each new interval is the previous one multiplied by an ease factor.

Anki’s default scheduler is a modified version of it.

Spaced repetition

A study method in which you review information at increasing intervals, usually with flashcards and an app that schedules each review.

Spacing effect

The finding that people remember information longer when they spread study sessions over time than when they spend the same time in one block.

Stability

In FSRS, the number of days it takes for the probability of recalling a card to fall from 100% to 90%. Each successful review increases it.

Read more: The FSRS algorithm

Testing effect

The finding that recalling information from memory makes it easier to remember later than studying it again for the same time.