The order and thickness of copper and dielectric layers in a board. Sets trace impedance, so it must match the fab's real material.
How it looks
Drawing from our real stackup (board file setup)
The real stackup of CORE3588, from its board file: PCB Power's standard 1.6 mm 4-layer build with 2116 prepreg. Notice how thin the glue layer under the top copper is (0.105 mm) compared to the core (1.2 mm).
How it works
Where it fits
The stackup is chosen at the constraints stage, together with the fab. It fixes how far each layer sits from the ground plane, which sets impedance.