RJ45 usually refers to a modular socket or plug with 8 positions (8 pins) defined by international connector standards, standardized by IEC (60) 603-7. Therefore, the RJ45 interface is also known as an 8j crystal head or crystal head, and the crystal head is a male head.

The RJ45 network interface is widely used for connecting network cables (referred to as Category 5 cables or twisted pair cables) between LAN and ADSL broadband internet users.