What is the difference between "solidus" and "slash"?

According to wikipedia,

The solidus ( ⁄ ) is a punctuation mark used to indicate fractions including fractional currency. It may also be called a shilling mark, an in-line fraction bar, or a fraction slash. (...)

The solidus is similar to another punctuation mark, the slash ( / ), which is found on standard keyboards; the slash is closer to being vertical than the solidus. These are two distinct symbols that traditionally have entirely different uses. However, many people no longer distinguish between them, and when there is no alternative it is acceptable to use the slash in place of the solidus.

So, unless you're engaging in some typography, saying 'slash' instead of 'solidus' should be completely acceptable (and probably more widely understood).

One authority which usefully distinguishes between the virgule (also called a slash) and the solidus is Robert Bringhurst's The elements of typography. He notes the higher verticality of the virgule, which he calls an "alternative form of the comma" (p.81). It's used as a general separatrix, in dates, and when naming alternatives (either/or) (though this last usage is frowned on by some). The more slanted solidus is used in fractions. Our difficulty is we only have one forward slash on our keyboards; you must resort to your "Symbols" catalog to find a solidus.