The Collaborative International Dictionary of English v.0.48:

Officer \Of"fi*cer\, n. [F. officier. See Office, and cf.
   Official, n.]
   1. One who holds an office; a person lawfully invested with
      an office, whether civil, military, or ecclesiastical; as,
      a church officer; a police officer; a staff officer. "I am
      an officer of state." --Shak.
      [1913 Webster]

   2. (U. S. Mil.) Specifically, a commissioned officer, in
      distinction from a warrant officer or an enlisted man.
      [1913 Webster]

   Field officer, General officer, etc. See under Field,
      General. etc.

   Officer of the day (Mil.), the officer who, on a given day,
      has charge for that day of the guard, prisoners, and
      police of the post or camp; abbreviated O. D., OD, or O.
      O. D.

   Officer of the deck, or Officer of the watch (Naut.), the
      officer temporarily in charge on the deck of a vessel,
      esp. a war vessel.
      [1913 Webster]