This document defines key number concepts including perfect squares, square roots, radicals, principle square roots, irrational numbers, real numbers, natural numbers, whole numbers, integers, and rational numbers. Perfect squares are whole numbers that are the product of a number multiplied by itself. The square root of a number is one of its two equal factors, with the radical sign used to represent a nonnegative square root. Irrational numbers cannot be expressed as a ratio of integers, while real numbers include both rational and irrational numbers.