Microworld
A small, self-consistent world where a powerful idea can be explored safely and exhaustively.
A microworld is a deliberately simplified domain — with its own objects, rules, and constraints — built so a learner can explore one powerful idea without the noise of the full world. Turtle geometry is a microworld for geometry. A Newtonian motion simulation where you nudge a "dynaturtle" is a microworld for physics.
The genius of a microworld is that it is complete and consistent: everything in it obeys its rules, so you can form a hypothesis, test it, and trust the result. Bugs become informative rather than catastrophic. You are free to be wrong, repeatedly, on the way to being right.
Microworlds are everywhere in good software learning: a spreadsheet, a sandbox game, a REPL, a unit-test harness. Each is a little universe where ideas can be safely run.