Max altitude

[darkroomsource]

Yes Keith, that's exactly it.

Another way of saying it: "The higher you go, the closer the stall speed is to the maximum speed of the aircraft."

EDIT:
I should not have said stall speed, what I meant was that the stall angle of attack changes due to the air density or lack there-of, such that the speed required to maintain flight without stalling gets closer and closer to the maximum speed the aircraft is capable of.