What is a "niche" in biology?

The niche that an organism occupies is the opening in the environment that the organism fills to make a living. A species' niche is its place in an ecosystem relative to the other organisms present.

Examples of niches found in nature include herbivores, which are adapted to extract energy from plants, and carnivores, which get their energy from eating herbivores. In a mature ecosystem, specialist niches arise within these broad categories. Some predators, for example, specialize in long-distance endurance running, while others specialize in ambush-style attacks on prey. Each of these is a unique approach to gathering food that different species adopt.