Compacts, are cameras with a non-interchangeable objective. Usually it’s a zoom objective, but there are some exceptions represented by compact cameras with fixed focal length. As their own name says, they have some pretty small dimensions and weight (even if there’s some exception), are easy to transport, even if, actually, the size can be pretty variable and there are very small ones and, at the same time, ones that are not really pocket-sized at all.
Until recently, it was easy to define compacts as cheap cameras, with small sensors and, consequently, with a low image quality, especially with poor light, but today it’s not like that anymore. Market has deeply changed and most of the people take pictures through smartphones they have in their pocket or bag, even those who never thought about taking pictures before. Nobody wants to buy a compact camera to take pictures anymore. This, made these cameras, that some brands don’t even have available anymore, actually disappear from the market.