In printing, register is the exact physical alignment of differing plates, colors, or front-to-back impressions. When a job is in register, the cyan, magenta, yellow, and black inks overlap perfectly to form a sharp image without blurry edges or color halos. A secondary, historic meaning refers to the inner alignment part of a typecasting mould.
Heard on the job
The magenta plate is slightly out of register, giving all the photos a fuzzy red shadow.