Pronunciation: (op'u-rā"ting),[key] —adj. 1. used or engaged in performing operations: an operating surgeon. 2. of, for, or pertaining to operations: an operating budget. 3. of or pertaining to the proper operation of a machine, appliance, etc.: a manual of operating instructions. 4. Railroads.of, pertaining, or belonging to railroad workers, as engineers or firemen, who are directly engaged in the mechanical operation of trains: an operating union.