Source : Webster's Revised Unabridged Dictionary (1913)

Viscountess \Vis"count`ess\, n. [F. vicomtesse, LL.
   vicecomitissa.]
   The wife of a viscount.

Source : WordNet®

viscountess
     n 1: a wife or widow of a viscount
     2: a noblewoman holding the rank of viscount in her own right