noun (n)

• a period of time spent in military service(noun.time)

  Synonym:
  duty tour, hitch, term of enlistment, tour, tour of duty
  

  source: wordnet30
• the act of enlisting (as in a military service)(noun.act)

  source: wordnet30
• The act or enlisting, or the state of being enlisted; voluntary enrollment to serve as a soldier or a sailor.(noun)

  source: webster1913