The Ebbinghaus curve is sometimes known as the forgetting curve.
It’s a theory about how memory fades - how our retention of new information is lost over time if we don’t put in effort to retain it.
Ebbinghaus ran some experiments on himself - he tried to memorize short nonsense words - and then tested his retention at later time intervals.
He remembered 100% immediately after learning, then he noticed he started forgetting really fast! In 20 minutes, 42% was gone. In 24 hours, 67%. In a month, 79% gone!
Here’s a chart of the Ebbinghaus forgetting curve.