Passage from Robert Recorde's 1557 book The Whetstone of Witte in which the equals sign "=" was introduced for the first time.

In symbols, the = symbol or “equals sign” represents an equivalence of the quantities or values on each side, left and right, of its position in a mathematical expression.

Etymology
In 1557 textbook The Whetstone of Witte, English mathematician Robert Recorde introduced the equals symbol “=” as a means of truncation of the words “is equal to”. [1] Specifically, Recorde states: [2]

“To avoid the tedious repetition of the words: is equal to, I will set a pair of parallels, or lines of one length, thus: =, because no two things can be more equal.”

References
1. Bodanis, David. (2000). E = mc² - a Biography of the World's Most Famous Equation (pg. 25). Berkley Books.
2. Recorde, Robert. (1557). The Whetstone of Witte: whiche is the seconde parte of Arithmeteke: containing the extraction of rootes; the cossike practise, with the rule of equation; and the workes of Surde Nombers. Publisher.

External links
● Equals sign – Wikipedia.
● The Whetsone of Witte – Wikipedia.