†(a) a military engagement (obsolete); (b) military service in the field, active service; frequently attributive in later use.

direct participation in military operations as a member of the armed forces; cf. active duty n.

(full-time) service in the operational work of the armed forces or police, esp. as opposed to service in a reserve or support capacity; also in…

full-time service with a regiment, as distinguished from service on reserve.