A stub is a skeleton program or routine used early in the software development process to substitute for a fully functional program that has not been developed yet. Stub programs are temporary sections of programming code that assist in testing interfaces to and from the complete program.

Minimal programming code is written in a stub. A stub must contain enough code to be compiled and linked with the other related programs as well as test data values being passed to and from other programs. To do this the stub must include at least the declarations of variables and functions. In addition programmers sometimes include simple functionality or text debugging messages that allow them to view the sequence of logic being executed by the stub.

As the development process proceeds, stub functions are replaced one at a time with the real functions. This allows the programmer to isolate errors, improving the quality of development.