Headphone Jack (3.5mm)
A small round connector for accepting the pin-shaped plug from a standard pair of music headphones.
Older phones used a smaller 2.5mm jack for phone headsets.
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.
3.5mm refers to the approximate diameter of the connector. 2.5 and 3.5 mm connectors look very similar, differing only in size.