Rotational velocity at the tips, usually. Depends on prop design, tip shape, thickness, rigidity of blade, etc. Chart of tip speeds on my website, link from main page. As velocity approaches .7 mach (very easy on 23" + props these days) the prop begins turning so fast that the blades' tips remain in turbulent air which begins a process called "cavitation". Cavitation of a prop means it is no longer "pulling" against the air, but just spinning. This creates a bizarre and very loud noise, usually, that some call "ripping".

"Ripping" is actually when a prop is nearly fully-cavitated at the tips because the rpms are such that the tips actually go supersonic, exceeding mach 1 velocity.

Generally, it is not recommended.