If you are young and living in Japan, chances are, either you or someone you know will be living in an apartment with something called a yunitto basu, which is just 'unit bath' pronounced with a Japanese accent. Now, what is a 'unit bath'? (If you have any plans of moving into an apartment in Japan, this little bit of information might prove to be useful.)
A 'unit bath' is a factory-produced bathroom module made of a single material. Because the ceiling, bathtub, and floor are made of a single continuous material, there's no concern for leakage. So far, this is the conventional definition of 'unit bath' as the expression is, or was, used in Japanese. Today, there is the additional, more specific implication of it including a bathtub, a sink, and a toilet in this single module. The picture above, posted by SaddaGocaraRupa on Flickr, gives a good idea of what I mean by this. —No, I don't know whether all 'unit baths' are necessarily this cramped, but the ones I've seen look almost exactly like this. This is especially a big deal in Japan, where (unlike back home) the bathtub and toilet are—or were—usually in separate rooms, no matter how small.
The merits of a 'unit bath,' I hear, include the fact that it's easy to clean because you can just shower down the entire room. …Sorry, I don't know what else; personally, I'm a staunch proponent of toilet-bathtub segregation, so it would be appreciated if someone who prefers 'unit baths' could leave a comment stating why. The only other reason I can imagine is that perhaps it brings the rent down, but I think it's the other way around, i.e., cheaper apartments tend to come with 'unit baths.' Cheap apartments with bathtubs and toilets in separate rooms are in fact nearly impossible to find if you restrict your options to more recently-constructed buildings.
So if you're looking for an apartment in Japan and hear that your prospective room has a 'unit bath,' you'll know what to expect. (At least as of 2007.)