Wiktionary
cherry pitter
n. A handheld device that removes pits from cherries.
Wikipedia
Cherry pitter
A cherry pitter is a device for removing the pit from a cherry, leaving the cherry relatively intact. Many styles of cherry pitter exist, including small tools held in the hand, domestic crank-operated machines with a hopper, and industrial machines.
Use of a hand-held cherry pitter involves first removing the stem, then pushing a retractable plunger through the cherry so that the stone is pushed out the other side.
A great homemade alternative to a cherry pitter is an unbent paperclip. Insert the large end into the cherry, then twist and pull out the pit.