I would guess that legibility is whether it is readable at all, where readability is how easy it is to read. Something that is 100% legible might still not be e

I would guess that legibility is whether it is readable at all, where readability is how easy it is to read. Something that is 100% legible might still not be easy to read - large blocks of 9pt text in a low-contrast grey colour on a white background for instance (and IMO).