Integral sign

The integral symbol:

is used to denote integrals and antiderivatives in mathematics.

The notation was introduced by the German mathematician Gottfried Wilhelm Leibniz in 1675 in his private writings; it first appeared publicly in the paper "De Geometria Recondita et analysi indivisibilium atque infinitorum" (On a hidden geometry and analysis of indivisibles and infinites), published in Acta Eruditorum in June 1686. The symbol was based on the ſ (long s) character, and was chosen because Leibniz thought of the integral as an infinite sum of infinitesimal summands.

The integral symbol is U+222B ∫ INTEGRAL in Unicode and \int in LaTeX. In HTML, it is written as ∫ (hexadecimal), ∫ (decimal) and ∫ (named "entity").

The original IBM PC code page 437 character set included a couple of characters ⌠ and ⌡ (codes 244 and 245, respectively) to build the integral symbol. These were deprecated in subsequent MS-DOS code pages, but they still remain in Unicode (U+2320 and U+2321, respectively) for compatibility.

The ∫ symbol is very similar to, but not to be confused with, the ʃ symbol (called "esh").

Related symbols include:

In other languages, the shape of the integral symbol differs slightly from the shape commonly seen in English-language textbooks. While the English integral symbol leans to the right, the German symbol (used throughout Central Europe) is upright, and the Russian variant leans to the left.

...
Wikipedia