Refers to hardware and software components that are linked together and dependent upon each other. For example, in a multiprocessing environment, where several computers share the workload, a tightly-coupled system would have to be shut down in order to add or replace a machine.

Software Tight Coupling
Tightly-coupled software means routines (modules, programs) that work in only one type of system and are dependent upon each other. For example, an operating system depends on its drivers to activate a peripheral device. Such drivers would require extensive programming changes to work in another environment. Contrast with loose coupling.