Literally means "visual type"; from a style/movement in J-Rock that started back in the 1980s, and became popular in Japan by the 90s. This style/movement has been largely credited to have been started by X Japan; which led to an uprising in the trend of the visual scene. It's characterized by bands (namely Japanese) that use dramatic costumes/hair/makeup to basically inhance their visual appearance. Although there are branches of Jrock, that have different classifications/characteristics that visual-kei. The style alone does not neseccarily have to do with the music; but more alone with its outlandish and unique clothing/hair/makeup-thats what its really known for.
Many visual-kei bands include: X Japan, Malice Mizer, Dir en Grey, Gazette, Phantasmagoria, Alice Nine, 12012, and many more to name.