What is the mathematical meaning for plus?

Answer 1

"Plus" can mean "super" as in names for petrol, but the mathematical meaning is "and add on", so 5 + 9 means "start with 5 and then add on 9". The sign also has the meaning "positive", as opposed to "negative" so when we say that the square roots of 9 are +3 and -3 we are indicating where these two numbers would lie on a number stretching from "negative number land" via zero to "positive number land".

Of course in the English language there are several other uses and meanings of "plus" as in the case of golfers wearing "plus fours", a type of trousers.