The quality of having plenty of room — the noun for what makes a place feel spacious. It is what you say when you want to talk about the feeling itself rather than describe the place: 'the spaciousness of the entrance hall', 'a sense of spaciousness'. Notice that it is about the impression as much as the measurement. A small flat with white walls, high windows and little furniture can have a real spaciousness; a large dark room crowded with cupboards has none. That is why architects, designers and writers like it. It is uncountable, it is a fairly formal and written word, and it very often turns up in the phrases 'a sense of spaciousness' and 'a feeling of spaciousness'. People also use it beyond rooms — of a landscape, a piece of music with long quiet gaps, or a life with unhurried time in it.