When you design a network, you've got to choose a set of protocols (which are basically sets of communication rules between machines), for each of various "layers" (which you can imagine as different abstraction levels, that network designers like to keep in mind when creating and combining protocols).

TCP and IP are protocols for two independent layers, that were created together and to be usable together; but can very well be used with other protocols. That happens fairly often: you can use IP along with a non-TCP protocol, or TCP along with a non-IP protocol.

The reason why TCP/IP is such a common abbreviation is that those two protocols have become the most widely used protocols in the Internet, and were key to its success.

When you design a network, you've got to choose a set of protocols (which are basically sets of communication rules between machines), for each of various "layers" (which you can imagine as different abstraction levels, that network designers like to keep in mind when creating and combining protocols).

Simpler version: protocols are like boxes in which we put our messages. Those boxes have different sizes, and you put your message in the smallest box, then the smallest box in a box that is a little bigger, etc. Choosing a set of protocols is choosing what kind of boxes you'll use, for each "layer" that surrounds your message.

TCP and IP are protocols for two independent layers, that were created together and to be usable together; but can very well be used with other protocols. That happens fairly often: you can use IP along with a non-TCP protocol, or TCP along with a non-IP protocol.

The reason why TCP/IP is such a common abbreviation is that those two protocols formed, together, the basis of the Internet and were key to its success.

(TCP and IP do have some functionalities that were designed specifically for them to function together, which is something purists often complain about, but they don't really prevent you from interfacing them with other protocols)