A two-seater is a thing you sit on — a car, a plane, a sofa — that is built for exactly two people. It is a noun: 'He drives a two-seater.' You can also use it before another noun, like an adjective: 'a two-seater sports car', 'a two-seater sofa'. Note the hyphen, and note that the word before 'seater' stays singular: two-seater, never 'twos-seater'. When people say 'a two-seater' with no other word, the thing they mean depends on where the conversation is. Talking about cars, it means a small sporty car with no back seats — and that carries a feeling with it: fun, a bit expensive, not for a family. Talking about furniture in a shop, it means a small sofa (British shops write 'two-seater sofa' on the label; in Taiwan and the US you would more often hear 'loveseat' for the same thing). Talking about small planes, it means a plane for a pilot and one passenger. The same pattern makes other words freely: three-seater, four-seater, and for buses or halls people jump to 'a fifty-seater'. So this is really a shape you can build with, not one fixed word to memorise. One more thing, though — 'two-seater' counts SEATS, not people who fit. A car sold as a two-seater is a two-seater even when a small child squeezes in somewhere. And a sofa sold as a two-seater is often really comfortable for one and a half.