The Collaborative International Dictionary of English v.0.48:

endangered species \en*dan"gered spe"cies\, n. sing. & pl.
   A species of plant or animal that has declined in numbers to
   a point where further irreversible decline and extinction[3]
   has a significant chance. Lists of endangered species are
   maintained by government agencies, and in many cases the
   killing of such species or destruction of their habitat is
   prohibited by law.

   Note: In the U. S. in recent years, development of certain
         tracts of land has been prohibited due to the
         likelihood that it will cause a reduction in the
         numbers of an endangered species. In some cases
         environmental organizations have initiated litigation
         to cause the courts to rule that a certain development
         plan is illegal due to a threat to an endangered
         species. Laws protecting endangered species have become
         in some cases a contentious political issue.
         [PJC]

WordNet (r) 3.0 (2006):

endangered species
    n 1: a species whose numbers are so small that the species is at
         risk of extinction