Overlay
Beginner
How well one layer lines up with the layer under it.
Novice
The misalignment between a layer and the layer it has to connect to, measured on special alignment marks. Too much overlay error leaves contacts missing their targets or shorts neighboring wires together.
Expert
A shared error budget: the scanner, the mask, the process and, on layers printed with several masks, the placement of each mask relative to the others all add to it. At advanced nodes even sub-nanometer misalignment can make a chip fail.