A vibration in which the tool cuts into waves left by the previous pass, feeding energy back into the system, so it grows instead of dying out. It sounds like a howl or buzz and leaves regular marks on the surface. Common cures are shortening tool stickout, changing rpm to break the resonance, increasing feed, using a sharper or variable-pitch tool, and stiffening the workholding.
Endmill's sticking out three inches, of course it's chattering, choke up on it and drop the rpm a couple hundred.
If chatter appears, try changing rpm first, both up and down; a small change often moves the cut out of the resonance.
Related terms
Feeds and speeds
The spindle speed and feed rate chosen for a cut, set from material, tool and machine.
Endmill
A rotating cutter with teeth on its end and sides, the main tool for milling slots, pockets and profiles.
Hog out
To remove a large volume of material fast with heavy roughing cuts, without worrying about finish.
Rubbing
A tool sliding over the work without taking a proper chip, usually because the feed is too light or the tool is dull.