1. Resources (in computing usually processing time or storage space) consumed for purposes which are incidental to, but necessary to, the main one. Overheads are usually quantifiable "costs" of some kind.
Examples: The overheads in running a business include the cost of heating the building. Keeping a program running all the time eliminates the overhead of loading and initialising it for each transaction. Turning a subroutine into inline code eliminates the call and return time overhead for each execution but introduces space overheads.
2. <communications> information, such as control, routing, and error checking characters, that is transmitted along with the user data. It also includes information such as network status or operational instructions, network routing information, and retransmissions of user data received in error.
3. Overhead transparencies or "slides" (usually 8-1/2" x 11") that are projected to an audience via an overhead (flatbed) projector.