The equal sign, equals sign, or "=" is a mathematical symbol used to indicate equality. It was invented in 1557 by Welshman Robert Recorde. The equals sign is placed between the things stated to be exactly the same, as in an equation. It is the Unicode and ASCII character 003D (in hexadecimal).
The "=" symbol that is now universally accepted by mathematics for equality was first recorded by Welsh mathematician Robert Recorde in The Whetstone of Witte (1557). The original form of the symbol was much wider than the present form. In his book Recorde explains his design of the "Gemowe lines" (meaning twin lines, from the Latin gemini):
...to auoide the tediouse repetition of these woordes : is equalle to : I will sette as I doe often in woorke use, a paire of paralleles, or Gemowe lines of one lengthe, thus: =, bicause noe .2. thynges, can be moare equalle.
...to avoid the tedious repetition of these words: "is equal to", I will set (as I do often in work use) a pair of parallels (or Gemowe lines) of one length (thus =), because no two things can be more equal.
However, a manuscript from the University of Bologna, dated from 1550–68, features the same equality symbol, possibly predating its usage by Recorde. According to Scotland's St Andrews University Maths History website:
The symbol '=' was not immediately popular. The symbol || was used by some and æ (or œ), from the Latin word aequalis meaning equal, was widely used into the 1700s.
The symbol "≡" (Unicode character 2261) is often used to indicate an identity, or a congruence relation in modular arithmetic. The symbol "≘" can be used to express that an item corresponds to another.
In programming languages, the equals sign may either denote a boolean operator to test equality of values (sometimes a double equal sign "=="), or it may denote an assignment (sometimes denoted with a colon-equals ":="). In some programming languages such as PHP a double equals sign ("==") denotes equivalence, meaning that the variables may not be of the same data type, but their values can be reduced to the same value. The triple equal sign ("===") denotes identity , meaning that not only do the two values parse to be the same, they are of the same data type. For instance, in PHP the expression ("0 == false") is true, but ("0 === false") is not, because the number 0 is an integer value, whereas false is a Boolean.