Animation technique where individual frames from live action footage are traced onto paper, used as a guide for naturalistic movement or as reference points for mattes.
One of the most famous early uses can be seen on the dancing skeletons in the 1932 Betty Boop cartoon Minnie the Moocher.
Used throughout the Disney blockbusters (Snow White was completely rotoscoped) and Ralph Bakshi infamously rotoscoped the entire cast in his version of The Lord of the Rings.

Also the name of the projecting machine used to trace the images.