The mathematical word plus has added various functions to its set since entering English from Latin in the 16th century. It can be a noun (statistical ability is a plus), a preposition (one week plus a day or two), an adjective (it’s plus 30° outside), and a conjunction (cycling’s a great way to stay fit, plus it’s good for you).
The last of these, used at the start of a sentence or independent clause and often followed by a comma, may also be described as an adverb (Plus, I wasn’t sure if you’d be there); authorities differ on the categorisation. The usage is controversial, receiving “considerable adverse comment” (MWDEU) and causing “widespread ripples of dismay among purists” (Robert Burchfield).
Posted by Stan Carey 
