The imaginary unit i is defined as the square root of which number?

1. Square roots of negative numbers are not real numbers. 2. To handle them, one new number is defined as the square root of negative 1, and it is named i. 3. Squaring that definition gives i squared equal to negative 1, which is what makes the system work. _Source: OpenStax Precalculus (CC BY 4.0), section 3.1 Complex Numbers_