1. A complex number is an ordered pair of real numbers written in the form a + ib, where a is the real part and b is the imaginary part. 2. Complex numbers combine real and imaginary numbers, extending the real number line to the two-dimensional complex plane with the horizontal axis for real parts and vertical axis for imaginary parts. 3. The fundamental operations of addition, subtraction, multiplication, and division are defined for complex numbers by applying the operations to the real and imaginary parts separately.