Design Nomenclature / Digital Model Making
Mask
Also called matte, layer mask, stencil, selection
A grayscale image that restricts where an operation applies — a per-pixel weight on an effect, not part of any image's own colour. Numerically identical to an alpha channel, put to a different job: alpha says what an image covers, a mask says where you are permitted to work.
In practice
Layer mask, clipping mask, vector mask, matte, selection, stencil — one number in six sets of clothes. Non-destructive editing is largely the discipline of putting the decision in a mask instead of into the pixels, because a mask can be argued with later.