A principle of cognitive science that states that learning is most effective at the outset of a study session, and that the effectiveness of study quickly declines the longer the session continues. In essence, the fresher the learner, the more effective the study session.
This tends to manifest itself in the learner having stronger retention of those concepts introduced at the beginning of a study session. This is also the basis for the proven efficacy of short, targeted study sessions distributed over time, rather than long, one-time trainings or cramming.
More about the primacy effect: