Forgetting Curve
The predictable decline of memory without review
The forgetting curve describes the predictable decline in recall when information is not revisited. The exact shape varies by material and person, but the qualitative point is stable: soon after learning, loss is rapid, then it slows.
Techniques like Spaced Repetition work by placing review near the steep part of the drop-off, and Retrieval Practice works by making that review active rather than passive.