The role a plant or animal plays in its community. What it eats, its predators, physical tolerances, etc define the niche of an organism. Two species cannot live stably in the same habitat if they occupy identical niches.
Is all of the physical, chemical and biological conditions required by a species for survival, growth and reproduction. Two further abstractions of this concept are the fundamental niche and the realized niche.
(nitch) [L. nidus: nest] • The functioning of a species in relation to other species and its physical environment.