Photomask
Beginner
A glass plate with a pattern on it. It works like a stencil to print one layer of a chip.
Novice
A quartz plate carrying the pattern of one layer, in chrome (or, for extreme-ultraviolet light, as a mirror pattern). A chip needs a full mask set, at least one per patterned layer, used one after another.
Expert
Written by electron-beam or laser writers from fractured data, measured for feature size and placement, inspected, repaired and covered with a pellicle. Multiple patterning splits one layer across several masks, a large part of why mask sets cost so much at advanced nodes.
Explained in GDS & tapeout (Design Flow).
See also: Mask data preparation (MDP), Optical proximity correction (OPC).