I never even thought about this. I guess I just always called it the “division symbol”. But lo, and behold, in my abstract algebra class today, my instructor couldn’t remember it, but was sure that it had a name. Thanks to Wikipedia, I now know it.
Obelus, like obelisk, is Greek in origin, from obelos, meaning a sharpened stick, spit, or pointed pillar. The plural is obeli.
Merriam Webster indicates that the primary definition of obelus is the symbol being used to mark a questionable passage in an ancient manuscript.
Wikipedia describes this use in Aristarchus’ markings in the works of Homer, and in the editing of the Bible by modern editors.
Wolfram MathWorld explains that it is also used in typography to indicate a footnote‡, although in The Elements of Typographic Style, the author, Robert Bringhurst, indicates that the dagger symbol †,
†, can also be called an obelus in such a context (page 306). He also says that this division sign can be called an obelisk (page 314).