Modular jack

A modular connector is an electrical connector that was originally designed for use in telephone wiring, but has since been used for many other purposes. Many applications that originally used a bulkier, more expensive connector have converted to modular connectors. Probably the most well known applications of modular connectors are for telephone jacks and for Ethernet jacks, both of which are nearly always modular connectors.

Modular connectors were originally used in the Registration Interface system, mandated by the Federal Communications Commission (FCC) in 1976 in which they became known as registered jacks. The registered jack specifications define the wiring patterns of the jacks, not the physical dimensions or geometry of the connectors of either gender. Instead, these latter aspects are covered by ISO standard 8877, first used in ISDN systems. TIA/EIA-568 is a standard for data circuits wired on modular connectors.

Other systems exist for assigning signals to modular connectors; physical interchangeability of plugs and jacks does not ensure interoperation, nor protection from electrical damage to circuits. For example, modular cables and connectors have been used to supply low-voltage AC or DC power and no clear standard exists for this application.

Modular connectors also go by the names "modular phone jack/plug", "RJ connector" and "Western jack/plug". The term "modular connector" arose from its original use in a novel system of cabling designed to make telephone equipment more modular. This includes the 4P4C handset connector.

It is very common to use a registered jack number to refer to the physical connector itself; for instance, the 8P8C modular connector type is often called RJ45 because the Registered Jack standard of that name was an early user of 8P8C modular connectors. A very popular use of 8P8C today is Ethernet over twisted pair, and that may be the most well known context in which the name RJ45 is known, even though it has nothing to do with the RJ45 standard. Likewise, the 4P4C connector is sometimes called RJ9 or RJ22 and various 6P connectors are called RJ11.

...
Wikipedia