In 1885, Hermann Ebbinghaus discovered the exponential nature of forgetting. We can roughly describe the forgetting rate by R = e(-t/S), where R is memory retention, S is the relative strength of memory, and t is time. Thus, strength of memory (S) is the best way to represent the forgetting rate.
For example, if you learn a foreign word today, you usually have only a 90% chance of remembering it after several days.
The forgetting rate tends to decrease after each repetition. Some strategies for learning aim to reduce forgetting by spaced repetition, by which the learned material is reviewed repeatedly. The spacing between the repetitions is adjusted so that the material is reviewed before it would have been forgotten.