1. In a program, a unit or section that can function on its own. In an integrated program, for instance, you can use the word processing module as though it were a separate, stand-alone program. From QUECID

2. A non-linked driver which contain device specific system routines and fills in non-boot gaps in the kernel. Modules can be loaded and removed dynamically and allow for smaller generic kernels. From Linux Guide @FirstLinux