EDIT The plus sign is the mathematical symbol for addition and known and used worldwide.

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EDIT
History
Before the plus and minus signs evolved, it was common in Italian and French mathematics to use the word "plus" for an addition. Sometimes that was shortened to a p with a horizontal line on top do identify it more clearly. The plus and minus we know today seem to have developed on a separate way, though.

Often, the Latin word "et" (and)was put between two numbers that were to be added. To write faster, this word was often reduced to a ligature very reminiscent of the + we know today. A symbol like that appears in the work "Algorismus proportionum" the manuscript of which was written around 1356 and 1361 by the mathematician Nicole d' Oresme (1323-1382) or a scribe.

In printed form, the + and - first appear in a book by Johannes Widman that was published in Leipzig in 1489: Behende und hüpsche Rechenung auff allen Kauffmanschafft (Mercantile Arithmetic). They are not used as algebraic symbols, however, but refer to surpluses and deficits in business transactions.

As symbols for operation, plus and minus first appear in 1514 in a book by Giel Vander Hoecke, pulished in Antwerp: "Een sonderlinghe boeck in dye edel conste Arithmetica"(Smith 1958, page 341). He was probably just following work by Henricus Grammateus, whose book was published later in 1518: "Ayn new Kunstlich Buech" (Cajori vol. 1, page 131).

In England, the plus and minus symbols where used before they appeared in print, for example to indicate whether barrels of wine were full. They came into general use as mathematical symbols after Robert Recordes used them in 1557 in "The Whetstone of Witte". Recorde wrote, "There be other 2 signes in often use of which the first is made thus + and betokeneth more: the other is thus made - and betokeneth lesse."

(Main source: Earliest Uses of Symbols of Operation)

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EDIT Use
- Math: Symbol for addition
- Positive pole on batteries
- in internatinal telephone numbers, the plus can replace the two zeros at the beginning:
0049 261 34 606 = +49 261 34 606

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EDIT Names
In company names, a plus indicates a partnership. It hints more towards a combination of two partners that are still widely separate (Gulf + Western), though, whereas a "&" is more of a real, equal partnership (Smith & Jones)

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EDIT Typography
A plus is a cross of two lines of equal length, place over each other in the middle at a right angle. It is used with a space before and after.
A plus is never oblique or italic in mathematical context, only in texts.

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EDIT Plus sign in other cultures
A Jewish tradition going back to at least the 19th century is to write a plus not as cross, but rather like an upside-down T. This practice is usually explained with the wish to avoid use of a symbol that's reminiscent of the christian cross. The variation of the plus is encoded separately in the Unicode at U+FB29 "Hebrew letter alternative plus sign"

Since the 1970s, this alternative plus has been taught in schools in Israel and is commonplace now in most elementary schools and some secondary schools. Books for adults use the international plus-sign though, apart from the works of some religious authors.

 jsw