Burr grinder
also: burr mill
A grinder that crushes beans between two abrasive burrs set a fixed distance apart, giving a uniform, adjustable grind.
A burr grinder crushes coffee between two hard, ridged surfaces called burrs. The gap between them sets the particle size, so every bean is broken down to roughly the same dimension. You adjust the gap to dial in coarse for French press or fine for espresso.
Why it matters: uniform particle size is the single biggest lever for good coffee. When particles are even, water extracts them at a similar rate, so you avoid the mix of over-extracted dust and under-extracted chunks that ruins flavor. This is why burr grinders are the quality standard and a blade-grinder is not (see burr-vs-blade).
Burr sets come in two main shapes, conical and flat, each with slightly different particle profiles. No grinder is perfect: all produce some fines and boulders, but a good burr grinder keeps the spread tight. For choosing a setting, see grind-size-guide.