Source : Webster's Revised Unabridged Dictionary (1913)

Plus \Plus\, a. [L., more; akin to Gr. ?, ?, and E. full. See
   {Full}, a., and cf. {Pi[`u]}, {Pleonasm}.]
   1. (Math.) More, required to be added; positive, as
      distinguished from negative; -- opposed to {minus}.

   2. Hence, in a literary sense, additional; real; actual.

            Success goes invariably with a certain plus or
            positive power.                       --Emerson.

   {Plus sign} (Math.), the sign (+) which denotes addition, or
      a positive quantity.

Source : WordNet®

plus sign
     n : a sign indicating the operation of addition