1 A piece of hardware or software that checks the outputs of a system while that system is operational. For a single channel system (i.e. no redundancy or diversity), the comparator might check across several outputs to see that only valid combinations are produced. The comparator may deal only with binary signals, usually termed voting logic, or may compare analog signals.
2 A piece of software that, for example, compares the contents of two text files and highlights any differences between the contents. It is often used in word processing or editing of program source files and as a software quality assurance tool in configuration management.