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.]
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.
WordNet (r) 3.0 (2006):
n 1: a sign indicating the operation of addition