plus sign

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Mathematics The symbol (+), as in 2 + 2 = 4, that is used to indicate addition or a positive quantity.
• noun Chemistry The same symbol, used to indicate dextrorotatory rotation of polarized light.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun arithmetic The symbol + used to denote the operation of addition and to indicate that a number is positive.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a sign indicating the operation of addition