Oxford English dictionary definition:
Definition of officer
noun
1a person holding a position of authority, especially one with a commission, in the armed services, the mercantile marine, or on a passenger ship: he is also a serving officer in the army
a policeman or policewoman: the RUC has 8,150 full-time officers tougher sentences for attacks on police officers
a bailiff.
2a holder of a public, civil, or ecclesiastical office:a probation officer the Chief Medical Officer
a holder of a senior post in a society, company, or other organization:a chief executive officer
3a member of a certain grade in some honorary orders, such as the grade next below commander in the Order of the British Empire.