The Lorenz attractor (also called Lorenz system) is a system of equations. Edward N. Lorenz formulated the equations as a simplified mathematical model for atmospheric convection. The equations are ordinary differential equations, called Lorenz equations. They are notable for having chaotic solutions for certain parameter values and starting conditions. In particular, the Lorenz attractor is a set of chaotic solutions of the Lorenz system which, when plotted, resemble a butterfly or figure eight.
The notion of butterfly effect is coupled with that of the Lorenz attractor. Lorenz first used the phrase in a speech he gave in 1972.