Technically it's a "title", however everyone refers to the XKCD text shown on hover as "alt text". (See http://m.xkcd.com/ http://m.xkcd.com/, even xkcd calls i

Technically it's a "title", however everyone refers to the XKCD text shown on hover as "alt text". (See http://m.xkcd.com/ http://m.xkcd.com/, even xkcd calls it that).