Given that there is not much difference between a <i>letter</i> and a <i>symbol</i> (especially in view of the two most upvoted answers), I propose:

• The letter "+" is used for the mathematical object commonly called <i>addition</i>.

• The letter "-" is used for the mathematical object commonly called <i>subtraction</i>.

• The letter "×" is used for the mathematical object commonly called <i>multiplication</i>.

Etc.