A monopoly means you're pretty much the only viable supplier/provider for a particular segment of the market. A dominant player is someone who isn't an outright

A monopoly means you're pretty much the only viable supplier/provider for a particular segment of the market. A dominant player is someone who isn't an outright monopoly, but nonetheless has a sizable market share and can influence pricing, competition, (i.e. by its patent licensing terms and deals with key component OEMs), the ability to set or influence standards etc.