Self-excited vibration between the tool and workpiece that leaves a rippled surface.
Chatter is regenerative: the tool leaves a wavy surface, which varies the chip thickness on the next pass, which makes the vibration worse. Once started it rarely stops on its own.
The cures target rigidity, not speed alone: shorten the tool overhang, support the workpiece, reduce radial engagement, or shift the spindle speed off the resonant frequency. Variable-helix cutters break the regeneration by making the tooth spacing irregular.
Trying to cure chatter by reducing the feed. It usually makes it worse — the tool rubs instead of cutting, which sustains the vibration.
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