Plain English Meaning
An instructor needs enough hours with a student, and enough different flying situations, to honestly judge whether the student is ready. A few quick lessons aren't enough to sign someone off.
Full Definition
In aviation instruction, the principle that an instructor must have sufficient contact hours and varied training situations with a student to reliably evaluate that student's knowledge, skill, judgment, and readiness before endorsing them for a checkride, solo flight, or other privilege.
Context Anchor
Seen in flight instructor training when discussing how students learn and why practice must be planned, not rushed.
Why Pilots Care
Insufficient time and opportunity leaves students with fragile understanding that can break down under real , increasing and prolonging training.