Either: a jacket around two or more cables which (in the initial state) adhere to each other on one site of the divider and are separated on the other one. So, the divider provides protection against unwanted separation of the undivided part of the cables, preventing cable salad. You often can move the divider along the cables to extend the length of the separated cables (the divider divides the adhering cables) or to join the separated cables earlier (which is not intended since the divider then doesn't provide protection for the undivided part of the cables!).
Or: a connector with one cable contact at one site and two or more cable contacts at the other side.
For instance, cable dividers are used for headphones, earphones, earbuds (TRS connector, aka, audio jack) and things you connect to your TV (RCA connector, aka, cinch).