The Collaborative International Dictionary of English v.0.48:

Countess \Count"ess\ (kount"?s), n.; pl. Countesses (-?s). [F.
   comtesse. See Count a nobleman.]
   The wife of an earl in the British peerage, or of a count in
   the Continental nobility; also, a lady possessed of the same
   dignity in her own right. See the Note under Count.
   [1913 Webster] Countinghouse

WordNet (r) 3.0 (2006):

countess
    n 1: female equivalent of a count or earl