a taxon is Endangered when it is not Critically Endangered but is facing a very high risk of extinction in the wild in the near future, as defined by any of the following criteria (A to E):

A) Population reduction in the form of either of the
following:

1) An observed, estimated, inferred or suspected
reduction of at least 50% over the last 10 years or
three generations, whichever is the longer,
based on (and specifying) any of the following:

a) direct observation
b) an index of abundance appropriate for the
taxon
c) a decline in area of occupancy, extent of
occurrence and/or quality of habitat
d) actual or potential levels of exploitation
e) the effects of introduced taxa, hybridisation,
pathogens, pollutants, competitors or
parasites.

2) A reduction of at least 50%, projected or
suspected to be met within the next ten years or
three generations, whichever is the longer,
based on (and specifying) any of (b), (c), (d), or (e)
above.

B) Extent of occurrence estimated to be less than 5000
km2 or area of occupancy estimated to be less than
500 km2, and estimates indicating any two of the
following:

1) Severely fragmented or known to exist at no
more than five locations.

2) Continuing decline, inferred, observed or
projected, in any of the following:

a) extent of occurrence
b) area of occupancy
c) area, extent and/or quality of habitat
d) number of locations or subpopulations
e) number of mature individuals.

3) Extreme fluctuations in any of the following:

a) extent of occurrence
b) area of occupancy
c) number of locations or subpopulations
d) number of mature individuals.

C) Population estimated to number less than 2500
mature individuals and either:

1) An estimated continuing decline of at least 20%
within 5 years or 2 generations, whichever is
longer, or

2) A continuing decline, observed, projected, or
inferred, in numbers of mature individuals and
population structure in the form of either:

a) severely fragmented (i.e. no subpopulation
estimated to contain more than 250 mature
individuals)
b) all individuals are in a single subpopulation.

D) Population estimated to number less than 250
mature individuals.

E) Quantitative analysis showing the probability of
extinction in the wild is at least 20% within 20
years or 5 generations, whichever is the longer.

[information taken from the IUCN homepage at http://www.iucn.org/themes/ssc/redlist/ redlist.htm]