EDIT The Solidus is a character used to separate alternatives or to replace the word »per« (miles/hour; also part of the percent-sign %). It is also a mathematical character.
EDIT History The name is Middle English, from Late Latin (nummus) solidus.
The first use as a mathematical character (division sign) is documented in ancient Egyptian numerical series from about. 2000 B.C.
EDIT Punctuation mark In medieval times in Europe, the solidus was used by scribes as a means of structuring sentences. It is a predecessor of our comma today (French virgule = comma).

EDIT Currency Sign Before the decimal notation was used in Great Britain, the solidus was used as an abbreviation for noting down monetary amounts:

3/6 = three shillings and six pence
10/- = ten shillings
£1/19/11 = one pound, nineteen shillings and eleven pence

EDIT Names
English: solidus, slash, virgule, oblique dash, forward slash, diagonal, stroke, separatrix, shilling mark, slant, Directory Seperator (UNIX, Java)

German :Schrägstrich, Trennungszeichen, Virgel

Italian: barretta

Dutch: schuine streep

French: virgule
EDIT Linguistics Used in verses to indicate linebreaks.

Good friend, for Jesus' sake forbear / To dig the dust enclosed here / Blest be the man that spares these stones / But cursed be he that moves my bones.
(William Shakespeare)
EDIT Diacritical Character In some languages, e.g. skandinavian languages, hte solidus indicate a special pronounciation of a character: Ø (slashed o, sounds like the »u« in »turn«).
EDIT Mathematics The Solidus is used as the division bar between numerator and denominator of an in-line fraction (Bringhurst, R. The Elements of Typographic Style, 2nd ed. Point Roberts, WA: Hartley and Marks, p. 284, 1997).
Today, it is frequently replaced by Fraction slash (U+2044) and Division Slash (U+2215)
EDIT Informatics The term »Forward slash« evolved with the advent of computer languages and the arising need to distinguish it from the »Backslash«.

It is part of the beginning of URLs, »http://«, and serves as a separator for the different levels of domain names: »http://decodeunicode.org/index.php«.

The solidus is not allowed as part of a file name.