A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers, and i is the imaginary unit (which satisfies the equation i2 = −1). In this expression, a is called the real part of the complex number, and b is called the imaginary part. This resource is suitable for 'A level,' students studying C1-C2. The lesson carefully explains the topic of complex numbers in an easy to follow manner.