The following is taken from the book Magic With Figures (subsequently published as Rapid Math Without a Calculator by A. Frederick Collins.
+ 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.