In your note - it would be "Hobbs Time" (engine running, regardless of rpm).
ORIGINAL: ggraham500
Hobbs time or tach time? Full size airplanes used to be rented by either Hobbs time (time engine turned on to engine turned off at any speed) or at tach time (engine running at 2500 rpm) or cruise speed. I believe for most small aircraft, 2500 rpm was the standard flight cruising rpm.