The following is taken from the book Short Cuts in Figures by A. Frederick Collins (originally published in 1916, later published as Magic With Figures and even later as Rapid Math Without a Calculator).
Other Signs Used in Arithmetic.—Besides the symbols used to denote the figures there are symbols employed to show what arithmetical operation is to be performed.
+ Called plus. It is the sign of addition; that is, it shows that two or more figures or numbers are to be added to make more, or to find the sum of them, as 5 + 10. The plus sign was invented by Michael Stipel in 1544 and was used by him in his Arithmetica Integra.
= Called equal. It is the sign of equality and it shows that the numbers on each side of it are of the same amount or are of equal value, as 5 + 10 = 15. The sign of equality was published for the first time by Robert Recorde in 1557, who used it in his algebra.
− Called minus. It is the sign of subtraction and it shows that a number is to be taken away or subtracted from another given number, as 10 − 5. The minus sign was also invented by Michael Stipel.
× Called times. It is the sign of multiplication and means multiplied by; that is, taking one number as many times as there are units in the other, thus, 5 × 10. The sign of multiplication was devised by William Oughtred in 1631. It was called St. Andrew's Cross and was first published in a work called Clavis Mathematicae, or Key to Mathematics.
÷ Called the division sign. It is the sign of division and means divided by; that is, it shows a given number is to be contained in, or divided by, another given number, as 10 ÷ 5. The division sign was originated by Dr. John Pell, a professor of mathematics and philosophy.