∫

Etymology
Gottfried Leibniz based the symbol on the Latin word, which he wrote ſumma (with a long s ⟨ſ⟩). This use first appeared publicly in his paper De Geometria, published in Acta Eruditorum of June, 1686, but he had been using it in private manuscripts since at least 1675.

Symbol

 * 1)  Denotes the integral over a given integrand, which follows this symbol.