In photography, a print of a whole bunch of negatives on a single sheet of photographic paper.
Especially when using 135 film, you can fit a whole roll
of pictures on a single 8"x10" sheet. The careful photographer will file the negatives (cut and stored in negative file pages) together with the proof sheet: this makes ordering reprints much easier.
Usually proof sheets are contact prints, even though you can special giant proof sheets on 16"x10" paper where the negative is twice its actual size.
It is always interesting to look at the proof sheets of great photographers; some negatives have never been printed (nor published), and you get a chance to see the selection process at work.