Merriam-Webster's Collegiate® Dictionary, 11th Edition
PLUS
I. adjective see: full
Date: 1579
algebraically positive, having, receiving, or being in addition to what is anticipated,
3. falling high in a specified range , greater than that specified, possessing a specified quality to a high degree, electrically positive, relating to or being a particular one of the two mating types that are required for successful fertilization in sexual reproduction in some lower plantlike organisms (as a fungus),
II. noun (plural pluses; also plusses)
Date: 1654
plus sign , an added quantity, a positive factor or quality, surplus ,
III. preposition
Date: 1668
increased by ; with the addition of , besides ,
IV. conjunction
Date: circa 1950
and , in addition to which ,
Usage: The preposition plus has long been used with a meaning equivalent to and (as in “two plus two”); it is not, therefore, very surprising that in time people have begun to use it as a conjunction much like and. Sense 2 is considered to be an adverb by some commentators. It is used chiefly in speech and in informal writing.