A groove or step on the tool face that curls the chip so it breaks into short pieces.
Long continuous chips are a genuine hazard: they tangle around the tool and the workpiece, cut operators, and carry heat back into the edge. A chip breaker controls this by forcing the chip to curl tightly enough that it fractures on its own.
Its geometry is matched to the depth of cut and feed it is meant for, which is why an insert designed for finishing will not break chips at roughing depths and vice versa. The ISO designation encodes the chip breaker in the fourth symbol.
Running an insert outside the depth-of-cut range its chip breaker was designed for, then blaming the insert for poor chip control. The break usually fixes it.
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