The Collaborative International Dictionary of English v.0.48:

Policeman \Po*lice"man\, n.; pl. Policemen.
   A member of a body of police; a constable.
   [1913 Webster]

WordNet (r) 3.0 (2006):

policeman
    n 1: a member of a police force; "it was an accident, officer"
         [syn: policeman, police officer, officer]

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "policeman":
   Dogberry, G-man, John Law, MP, bailiff, beadle, beagle, bluebottle,
   bluecoat, bobby, bound bailiff, bull, captain, catchpole,
   chief of police, commissioner, constable, cop, copper, deputy,
   deputy sheriff, detective, fed, federal, flatfoot, flic, fuzz,
   gendarme, government man, gumshoe, heat, inspector, lictor,
   lieutenant, mace-bearer, man, marshal, mounted policeman, nab,
   narc, officer, paddy, patrolman, peace officer, peeler, pig,
   police, police captain, police commissioner, police constable,
   police inspector, police matron, police officer, police sergeant,
   policewoman, portreeve, reeve, roundsman, sergeant,
   sergeant at arms, sheriff, superintendent, tipstaff, tipstaves,
   trap, trooper