Forgetting Curve
The steep drop in memory right after learning. Most forgetting happens in the first day, then slows.
The forgetting curve, first measured by Hermann Ebbinghaus in the 1880s, shows that memory for new material drops steeply within hours and days of learning, then levels off. Without review, most of a day's new vocabulary is gone within a week.
The encouraging half of the finding: each successful review flattens the curve. After a handful of well-timed retrievals, a word's curve becomes nearly horizontal, which is the practical meaning of "learned." This is why daily small reviews outperform weekend marathons.