Roger, NO IT IS NOT!

A cartridge can fit in a chamber PERFECTLY, and it still has headspace. Also, depending on the style case, headspace on a cartridge can be 2". That would be some movement inside a chamber.

EXCESS headspace in a particular rifle is the distance a PERFECTLY sized cartridge moves in a rifle chamber. FWIW, Dutch.

I'm always tickled when this comes up and the complicated factoids it draws, when in fact it's so damned simple.

Load a factory round in any rifle. Hold the rifle barrel to the sky. Turn the rifle barrel to the earth. The round will fall until something stops it. Point the muzzle back to the sky. The round will fall until something stops it. The distance the round falls is head space.

Dutch,
That is the way people talk.
That is the third usage.