The Collaborative International Dictionary of English v.0.48:

Heiress \Heir"ess\, n.
   A female heir.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heiress
    n 1: a female heir [syn: heiress, inheritress, inheritrix]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "heiress":
   apparent heir, beneficiary heir, child, coparcener, daughter,
   descendant, fideicommissary heir, fiduciary heir, foster child,
   grandchild, granddaughter, grandson, heir, heir apparent,
   heir expectant, heir general, heir in tail, heir of entail,
   heir of inventory, heir of line, heir portioner, heir presumptive,
   heir whatsoever, heritor, inheritor, inheritress, inheritrix,
   joint heir, next in line, offspring, presumptive heir,
   remainderman, reversioner, scion, son, son and heir, sonny,
   stepchild, stepdaughter, stepson, successor



Bouvier's Law Dictionary, Revised 6th Ed (1856):

HEIRESS. A female heir to a person having an estate of inheritance. When
there is more than one, they are called co-heiresses, or co-heirs.