Imaginary numbers: numbers that include the unit i, defined by the property i^2 = -1, allowing square roots of negative numbers to be expressed. They cannot be represented on the real number line and instead form part of the complex number system, where numbers are written in the form a + bi with real part a and imaginary part b. Imaginary numbers are essential for solving equations with no real solutions and are widely used in advanced mathematics, physics, and engineering.
