The Collaborative International Dictionary of English v.0.48:

Plus \Plus\, a. [L., more; akin to Gr. ?, ?, and E. full. See
   Full, a., and cf. Pi[`u], Pleonasm.]
   [1913 Webster]
   1. (Math.) More, required to be added; positive, as
      distinguished from negative; -- opposed to minus.
      [1913 Webster]

   2. Hence, in a literary sense, additional; real; actual.
      [1913 Webster]

            Success goes invariably with a certain plus or
            positive power.                       --Emerson.
      [1913 Webster]

   Plus sign (Math.), the sign (+) which denotes addition, or
      a positive quantity.
      [1913 Webster]

WordNet (r) 3.0 (2006):

plus sign
    n 1: a sign indicating the operation of addition