Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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