A small round connector for accepting the pin-shaped plug from a standard pair of music headphones.

While 2.5mm is the standard for phone headsets, the slightly larger 3.5mm size is the standard for music headphones and just about every other type of electronic device other than phones (music players, PCs, etc.)

See: 2.5mm Headset Jack

Either size can support stereo sound and/or a microphone, depending on the number of separate connector rings on the plug or jack.