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
    adj 1: on the positive side or higher end of a scale; "a plus
           value"; "temperature of plus 5 degrees"; "a grade of C
           plus" [ant: minus]
    2: involving advantage or good; "a plus (or positive) factor"
       [syn: plus, positive]
    n 1: a useful or valuable quality [syn: asset, plus] [ant:
         liability]
    2: the arithmetic operation of summing; calculating the sum of
       two or more numbers; "the summation of four and three gives
       seven"; "four plus three equals seven" [syn: summation,
       addition, plus]

The Free On-line Dictionary of Computing (19 January 2023):

PLUS

   Late 60's.  Machine-oriented systems language used internally
   by Univac.


The Free On-line Dictionary of Computing (19 January 2023):

plus
+

    "+", ASCII character 43, 0x2B.  The mathematical
   symbol for the addition operator, also used with the same
   meaning in arithmetic expressions in nearly all programming
   languages.

   Common names: ITU-T: plus; add.  Rare: cross; INTERCAL:
   intersection.

   In programming, the operator is sometimes overloaded to
   perform other tasks like concatenating strings.  In the C
   language and its many imitators, the symbol is doubled, as in
   "x++" or "++x" to give an increment operator that adds one to
   its operand ("x" in this case) and also returns x's previous
   or resulting value respectively.

   In a regular expression, "+" means match one or more
   instances of the previous pattern.  Thus /b(an)+a/ would match
   any of "bana", "banana", "bananana", etc. (see banana
   problem).

   (2010-03-20)