In mathematics, "i" is the symbol for the imaginary unit, which is defined as the number whose square is equal to negative 1. Imaginary numbers are used to extend the real numbers to what are called complex numbers, using addition and multiplication. The value of "i" is equal to the square root of negative 1, which is an imaginary value. Imaginary numbers are important in mathematics and have applications in fields such as science, quantum mechanics, and relativity.