A 'niche' (pronounciation as in French: neash) is a small, specialised segment of the market.
Niche marketing means that you offer a product or service, not for the mainstream, but for a special 'sub-population'. As a strategy, niche marketing means that your goal is to be a big fish in a small pond, instead of a small fish in a big pond.
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Deaf people, hearing impaired people, sign language users: all of these are 'niche markets'.
Deaf entrepreneurs have to decide what their market is: deaf people? sign language users? people with disabilities? Or a 'hearing' niche market, for instance parents of young kids (baby sign?), art lovers (photography?), etc.
For Deaf entrepreneurs, the Deaf 'niche' market may be easier, because it is homeground: you know the target group, your Deafness is an advantage, and communication is not a problem. The downside: it is a very small market, in most countries.
Mainstream markets, even mainstream 'niche' markets are usually larger. The downside: communication may be a problem; your deafness may be a disadvantage; you may have to consult or hire hearing people to help you target and reach your market.