The Collaborative International Dictionary of English v.0.48:

Manhole \Man"hole`\, n.
   A hole through which a man may descend or creep into a drain,
   sewer, steam boiler, parts of machinery, etc., for cleaning
   or repairing.
   [1913 Webster]

WordNet (r) 3.0 (2006):

manhole
    n 1: a hole (usually with a flush cover) through which a person
         can gain access to an underground structure

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "manhole":
   air hole, armhole, blowhole, bullet-hole, bunghole, cringle,
   deadeye, eye, eyelet, gasket, grommet, guide, keyhole, knothole,
   loop, loophole, mousehole, peephole, pigeonhole, pinhole, placket,
   placket hole, porthole, punch-hole, spiracle, tap, vent,
   venthole