Plain English Meaning
When you learn something new and slot it neatly into what you already know, that's assimilation. The new piece fits the existing picture, so the picture itself doesn't have to change.
Full Definition
In cognitive learning theory, the mental process by which a learner takes in new information and fits it into knowledge structures they already have, without changing those existing structures.
Context Anchor
Seen in instructor training when discussing how students understand new aviation ideas, procedures, and skills.
Derivation
From the Latin 'assimilare', meaning 'to make similar.' The idea is that the learner makes new information 'similar to' what they already understand, so it can be filed alongside it.
Why Pilots Care
Instructors use this concept when sequencing lessons. If new material can be assimilated into what a student already knows, is faster and sticks better. If it can't, the instructor has to help the student rebuild their first.