In electronics, a jack is a socket.

In some cases it is arbitrary which of a pair of mating connectors is the plug, and which the socket. See plug.

The term jack occurs in several related terms:

• In the telephone jack of old-style manual telephone exchanges, which is the socket fitting the original 1/4" jack plug, also known as a phone jack.

• In the RJ in RJ11, RJ45 and similar which stands for registered jack, see RJ-XX.

• In the name RCA jack, also known as a phono jack or 1/4" phono jack.

When used with a size, the term jack refers to the socket that matches the corresponding size of jack plug, as in 6.5mm jack, 6.3mm jack, 1/4" jack, 3.5mm jack, miniature jack, 2.5mm jack, subminiature jack, and similarly for non-standard sizes.

A headphone jack or earphone jack is normally one of the three standard sizes of jack, but the term could refer to any socket used for this purpose.