Control joint (contraction joint)
A deliberately weakened line in a slab, cut or tooled to a depth that forces shrinkage cracking to occur there instead of randomly across the surface.
What does control joint (contraction joint) mean?
Control joints do not prevent cracking; they decide where it happens and keep it straight and inconspicuous. They must be deep enough relative to slab thickness to actually create a weak plane, and they must be spaced closely enough for the slab thickness and mix. They also need to be placed early enough in the concrete's life to beat the cracking they are meant to control.
In practice
Cracks that appear a few inches away from a joint usually mean the cut was too shallow or made too late.