: all that is authorized, required, or normally associated with some field of responsibility (as of a policeman, fireman, or soldier) — used especially in connection with assessment of responsibility for or classification of sickness, injury, or death of persons subject to a line of duty
it is now customary to consider any sickness or injury of a member of an armed service that is suffered while on active duty to have been incurred in the line of duty in the absence of personal fault or neglect or of existence of the condition prior to entry into service