RJ
Registered Jack

Telephone jacks are also known as registered jacks, sometimes described as RJ-XX, and are a series of telephone connection interfaces that are registered with the U.S. FCC (Federal Communications Commission). The term jack sometimes means both receptacle and plug or just the receptacle. The most common telephone jack is the RJ-11, this has six conductors but usually is implemented with four. The RJ-45 is a single-line jack for digital transmission using either untwisted or twisted wire. The interface has eight pins, and is commonly used for ISDN and Ethernet connections.