detective

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person, usually a member of a police force, who investigates crimes and obtains evidence or information.
• adjective Of or relating to detectives or their work.
• adjective Suited for or used in detection.

from The Century Dictionary.

• Fitted for or skilled in detecting; employed in detecting: as, the detective police.
• Relating to detectives or to detection: as, a detective story.
• noun A person whose occupation it is to discover matters as to which information is desired, particularly concerning wrong-doers, and to obtain evidence to be used against them.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who business it is so detect criminals or discover matters of secrecy.
• adjective Fitted for, or skilled in, detecting; employed in detecting crime or criminals.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun law enforcement A police officer who looks for evidence as part of solving a crime; an investigator.
• noun A person employed to find information not otherwise available to the public.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an investigator engaged or employed in obtaining information not easily available to the public
• noun a police officer who investigates crimes