A specialized or narrowly targeted market category.
Primarily refers to the sub categories of a specific market. Example of a market is Groceries, while a niche in that industry would be Organic, or Vegetables.
In the Adult Industry, there are two markets, one is called 'mainstream' and the other is naturally the 'gay' market. Mainstream Market is heterosexual.
Originally a term used to describe a species use of specific foods, and environment. It was primarily coined by ecologists. It has since been used in the business community to determine a highly targeted or specialized area within a larger market.
It can also refer to a person finding their place in the wheels of industry. 'They found their niche'
History (Modern): Today's use is more about related products or actions within a defined larger category. In porn, there are many various sexual acts that are considered 'niches'. What makes them a 'niche' is that they appeal to only a small select group, not to everyone.
While Oral Sex is a niche, it has sub niches. Hand Job versus Blow Job for example. Rimming versus Dildo Play or Intercourse can be niches within the larger Niche of Anal Sex.
Niche is not an act, but a grouping of categories. In the Gay Market, some of the more common niches are Oral, Anal, Leather, Truckers, Orgy, Bareback, Condom Sex, Twinks, Bears, Bondage, Muscle, Jocks, Frat Boys, Cowboy, Asian, European, Russian, Uniform.
As shown, there are niches within niches as well. Prime example is Twink which can also include Jocks, and Frat Boys.
Noteworthy: In Porn, niches are generally created or developed by persons who have a distinct interest in that particular category. Usually a field not highly touted by larger porn companies.
A sub category in UK Garage Music circles; ecological niche, referring to the position of a species in an eco system; an architectural reference.