Chaos theory is the study of systems that are highly sensitive to slight, even imperceptible changes in initial conditions. This is popularly known as the butterfly effect. Many natural systems exhibit chaotic behavior, including weather and electron orbitals.
Chaos was summarized by Edward Lorenz, who is now recognized as a pioneer of Chaos theory, thusly:
Chaos: When the present determines the future, but the approximate present does not approximately determine the future.