What does "officer" mean?

Officer: (U. S. Mil.) Specifically, a commissioned officer, in distinction from a warrant officer. 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 quard, prisoners, and police of the post or camp. -- 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.