The general, predictable pattern of the process of forgetting learned information.
Psychologists have been interested in the processes of learning and forgetting since the early days of the discipline. The researcher who pioneered this field, Hermann Ebbinghaus (1850-1909), invented the nonsense syllable in order to be able to assess "pure" learning, that is, learning free of meaning, and the rate at which we forget. He served as his own subject and learned an incredible number of lists of nonsense syllables. He used material with little or no meaning because he was aware that learning new information is influenced by what we already know. He decided to create learning situations that were free of prior knowledge.
The way that we forget is highly predictable, following what psychologists call the forgetting curve. When we acquire knowledge, much of our forgetting occurs right away. Ebbinghaus discovered that a significant amount of information was forgotten within twenty minutes of learning; over half of the nonsense material he learned was forgotten within an hour. Although he forgot within a day almost two thirds of the material he learned, retention of the material did not decline much beyond that period. In other words, if information is retained for a day, the knowledge was there to stay.
Ebbinghaus's forgetting curve is actually much more dramatic than a forgetting curve would be for meaningful material. When the learner is able to connect new information with old information, he still might forget what was learned, but the amount and speed of forgetting is likely to be less than what Ebbinghaus experienced.
See also Ebbinghaus, Hermann