Aircraft Characteristics Practice Test

Absolute ceiling is the highest altitude at which an airplane can maintain level flight with the power available; at that point the rate of climb is zero. In other words, you can climb up to this altitude, but beyond it you cannot gain altitude because there isn’t enough excess thrust to overcome the increasing drag and thinning air. That’s why the phrase “the highest altitude with zero climb” fits the concept—the aircraft can’t climb any higher with the power it has.

This isn’t about the speed you can reach at altitude, nor about drag becoming zero (drag never vanishes in normal flight), and it isn’t defined as the altitude with zero thrust.