A notch or groove located on the lower front side of the metal shank of individual foundry type characters. Compositors use the nick as a tactile and visual guide to ensure letters are placed right-side up in the composing stick. Different typefaces or point sizes often feature unique combinations of nicks, making it easy to spot a wrong font mixed into a line.
Heard on the job
You can tell there's a wrong font in that line because the nick on the lowercase 'e' doesn't line up with the rest.
Insider tip
A compositor rarely looks at the face of the type while setting; they feel for the nick with their thumb.