Plain English Meaning
The training a pilot does when stepping up or across to a different kind of airplane than they are used to flying. It teaches them how the new airplane works, how it handles, and how to operate it safely.
Full Definition
Structured training a pilot completes when moving to an aircraft that has performance, systems, or handling characteristics significantly different from those they are already qualified to fly. It covers the new aircraft's systems, procedures, limitations, and flight characteristics through ground instruction and flight instruction, with the goal of building competence and safety in the new type before flying it as pilot in command.
Context Anchor
You see this term when a pilot is moving from one airplane or airplane setup to another, such as from a training airplane to a faster, heavier, or more .