Fork

In the context of source code, a fork refers to someone taking a copy of the code off in their own direction.

It's like a source versioning branch, but

frequently to add features that the original project isn't focusing on

often not with direct and shared intent to keep them in sync (but there are parallel projects that very much do)

usually stored somewhere else

In the context of *nix-style operating systems the fork() syscall is a way to create/copy a process)