Hi Brian, RJ stands for “Registered Jack”.The “45” in RJ45 refers to the number of the interface standard. An RJ45 connector has eight pins. This means an RJ45 cable has eight copper wires.
There is an excellent article written by one of our experts Ms. Angelina about these RJ45 connectors. You may refer to this article by clicking on the link: Deciphering Male and Female RJ45 Connector