Dual Numbers
It is like a complex number but instead of an i (imaginary number) we have an \(\epsilon\) having the properties of \(\epsilon^2 = 0\) and it commutes freely with real numbers. So a dual number has a real part and a dual part, like \(5 + \epsilon \, 6\).
\(\epsilon\) can also be thougth as a Dual Operator.
All sorts of interesting stuff arrives from this simple property, like Dual Angles.