any of the positive or negative integers, or any of the set of all real or complex numbers, the latter containing all numbers of the form a+bi, where a and b are real numbers and i denotes the square root of –1. (Numbers of the form bi are sometimes called pure imaginary numbers to distinguish them from “mixed” complex numbers.) The real numbers (real number) consist of rational and irrational numbers. Rational numbers, such as 12, ¹³/₅, or –⁴/₁₁, are those numbers that can be expressed as integers or as the quotient of integers, whereas the irrational numbers (irrational number), such as √2, are those that cannot be so expressed. All rational numbers are also algebraic numbers (algebraic number)—i.e., they can be expressed as the root of some polynomial equation with rational coefficients. Although some irrational numbers, such as √2, can be expressed as the solution of such a polynomial equation (in this case, x²=2), many cannot. Those that cannot are called transcendental numbers. Among the transcendental numbers are e (the base of the natural logarithm), π, and certain combinations of these. The first number to be proved transcendental was e (by Charles Hermite (Hermite, Charles) in 1873), and π was shown to be transcendental in 1882 by Ferdinand von Lindemann (Lindemann, Ferdinand von).

Other classes of numbers include square numbers—i.e., those that are squares of integers; perfect numbers (perfect number), those that are equal to the sum of their proper factors; random numbers, those that are representative of random selection procedures; and prime numbers, integers larger than 1 whose only positive divisors are themselves and 1.