An equation is a mathematical statement asserting the equality of two expressions, connected by the equals sign (=).
Equations contain variables (unknown values) and constants. Solving an equation means finding all values of the variables that make the statement true. Equations can be linear, quadratic, polynomial, exponential, trigonometric, or differential. Equations model relationships in science, engineering, economics, and everyday problem-solving.
The equals sign (=) was invented by Welsh mathematician Robert Recorde in 1557, who said "noe 2 thynges can be moare equalle" than two parallel lines. Ancient Egyptian and Babylonian mathematicians solved equations using rhetorical methods. Al-Khwarizmi's 9th-century work on "al-jabr" gave us systematic equation-solving methods. Modern equation notation developed over the 16th-17th centuries.