Same thing that makes a guitar a guitar, a mandolin a mandolin, etc. etc. Most stringed instruments, if not all, have many alternate tunings. In general though, a given instrument fits a specific set of dimensions, give or take very little. A uke is based on both it's size and scale, including scale length, and the number of strings.
i.e. A tiple is often the exact same size as a concert or tenor uke, but it has 10 strings....it is not a 10 string uke. Ukes in general have 4, 5, 6, and 8 strings within the framework of a ukulele scale. Taropatch ukes are generally concert size as opposed to the general term "8 string uke" for tenors, banjo ukes are banjos with a ukulele scale and tuned like a uke. A U-Bass is a ukulele sized bass, as it has a bass scale, if not the length of a standard bass and is not ukulele. Etc. etc. etc. Just when you think you have it, someone makes a variant, then you have to ask is it a ??????????? or a ???????????