operation /ˌɒpərˈeɪʃən/ noun [C]
MEDICAL TREATMENT › when a doctor cuts someone's body to remove or repair part of it: a heart/lung operation a major/minor operation My son's got to have an operation.
ORGANIZATION › an organization or business: a large commercial operation
ACTIVITY › an activity that is intended to achieve a particular purpose: a military/peacekeeping operation a joint operation by French and Spanish police an operation to smuggle drugs into Britain
in operation › If a machine or system is in operation, it is working or being used: The new rail link is now in operation. Most of the machines are now back in operation.