Final answer:
The set of numbers in the form a+bi is known as complex numbers; it is a complex number if b is non-zero and a real number if b is zero.
Step-by-step explanation:
The set of all numbers in the form a+bi, where a and b are real numbers, is called the set of complex numbers. If b≠0, then the number is also called a complex number. If b=0, then the number is also called a real number. Essentially, every complex number can be represented on a complex plane, with a being the real part and bi being the imaginary part. When b is not zero, the number has both real and imaginary components. When b equals zero, the number lies on the real axis and is therefore a real number.