Modular connector is the name given to a family of electrical connector

Electrical connector

An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...

s originally used in telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 wiring and now used for many other purposes. Many applications that originally used a bulkier, more expensive connector have now migrated to modular connectors. Probably the most well known applications of modular connectors are for telephone jacks and for Ethernet

Ethernet

Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 jacks, both of which are nearly always modular connectors.

Modular connectors were originally used with the registered jack

Registered jack

A registered jack is a standardized physical network interface — both jack construction and wiring pattern — for connecting telecommunications or data equipment to a service provided by a local exchange carrier or long distance carrier. The standard designs for these connectors and their wiring...

 system, which precisely describes how the connectors are wired for telecommunications.