a perforated metal plate situated behind the faceplate of a color television picture tube and having holes aligned to insure that each of three electron beams strikes only its corresponding red, green, or blue phosphor dot.
Oddly enough, the random nature of the shadow mask is the key to the whole operation.
Several recipes were tested to optimize the figures of merit listed above using a silicon shadow mask.
British Dictionary definitions for shadow mask
(television) a perforated metal sheet mounted close to the phosphor-dotted screen in some colour television tubes. The holes are positioned so that each of the three electron beams strikes the correct phosphor dot producing the required colour mixture in the image