In ecology, the smallest unit of a habitat that is occupied by an organism. Habitat niche refers to the physical space occupied by the organism; ecological niche refers to the role it plays in the community of organisms found in the habitat. The activities of an organism and its relationships to other organisms are determined by its particular structure, physiology, and behaviour. Two organisms that occupy similar niches but that are native to different geographical areas are termed ecological equivalents.
Source: Encyclopaedia Britannica 2002