Definition:Imaginary Number/Historical Note

Historical Note on Imaginary Number
was one of the first to accept negative numbers, and possibly the first to consider their square roots, which he did in his.

When considering the roots of $x^2 + 40 = 10 x$, and determining that they are $5 \pm \sqrt {-15}$, he concluded:
 * These quantities are "truly sophisticated" and that to continue working with them would be "as subtle as it would be useless".

As negative numbers were even then considered "false" and "fictitious", no wonder the square roots of negative numbers would be named imaginary.

, while happy to accept negative numbers, wrote of complex numbers:
 * These Imaginary Quantities (as they are commonly called) arising from the Supposed Root of a Negative Square (when they happen) are reputed to imply that the Case proposed is Impossible.

used $\sqrt {-1}$ with confidence, and published what is now known as Euler's Identity:
 * $e^{i \pi} + 1 = 0$

and introduced the letter $i$ to mean $\sqrt {-1}$.

Subsequently, and  all (independently) had the idea of plotting the real part and imaginary part of a complex number on the plane.

Final acceptance of imaginary numbers was complete when interpreted a complex number as an ordered pair and defined its properties axiomatically.