Business / Search Engine Optimization (SEO) / Niche: A topic or subject which a website is focused on. Search is a broad field, but as you drill down each niche consists of many smaller niches. Generally it is easier to compete in small, new, or underdeveloped niches than trying to dominate large verticals. As your brand and authority grow you can go after bigger markets.
Science / Biology / Ecological Niche: The role an organism occupies and the function it performs in an ecosystem; closely associated with feeding. MORE
Science / Biology / Niche Overlap: The extent to which two species require similar resources; specifies the strength of the competition between the two species. MORE