The Collaborative International Dictionary of English v.0.48:

Detective \De*tect"ive\, a.
   Fitted for, or skilled in, detecting; employed in detecting
   crime or criminals; as, a detective officer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Detective \De*tect"ive\, n.
   One who business it is so detect criminals or discover
   matters of secrecy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

detective
    n 1: a police officer who investigates crimes [syn: detective,
         investigator, tec, police detective]
    2: an investigator engaged or employed in obtaining information
       not easily available to the public

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "detective":
   Bow Street runner, FBI, FBI agent, Federal, G-man, MP,
   Secret Service, Sherlock, Sherlock Holmes, T-man, asker, bailiff,
   beadle, beagle, bound bailiff, captain, catchpole, catechist,
   chief of police, commissioner, constable, cop, copper,
   cross-interrogator, cross-questioner, deputy, deputy sheriff, dick,
   examiner, eye, fed, federal, flic, gendarme, government man,
   gumshoe, hawkshaw, hotel detective, house detective, house dick,
   inquirer, inquiry agent, inquisitionist, inquisitor, inspector,
   interlocutor, interpellator, interrogator, interrogatrix,
   interviewer, investigator, lictor, lieutenant, mace-bearer,
   marshal, mounted policeman, narc, officer, operative,
   opinion-sampler, patrolman, peace officer, peeper, plainclothesman,
   police captain, police commissioner, police constable,
   police detective, police inspector, police matron, police officer,
   police sergeant, policeman, policewoman, poller, pollster,
   portreeve, private detective, private eye, private investigator,
   prober, querier, querist, questioner, questionist, quizzer, reeve,
   revenuer, roundsman, sampler, secret agent, sergeant,
   sergeant at arms, sheriff, sleuth, snoop, snooper, store detective,
   superintendent, tec, tipstaff, tipstaves, treasury agent,
   trooper