Why is $i$ called "imaginary"?

I was reading this question, and, after reading the responses, I felt like I had a much better understanding about how they're just another type of number definition.

Why, then, are they called imaginary? I know there's some arbitrariness to why anything is called anything, but this name in particular probably leads a lot of people to be stuck on the question the poster of the linked questions asked. Almost as if the name itself it trying to encourage us not to trust this crazy "number."

Googling was able to get me that Descartes coined the term, but I couldn't find anything on why, or why the name stuck, even in translation.

• because they did not have math.SE back then and these answers you have read were not available to them. It was simply mind boggling without the tools that were developed over time only later.
– user13838
Commented Oct 14, 2011 at 2:15
• Have you seen this? BTW, nice question. :) Commented Oct 14, 2011 at 2:15
• You may as well ask why $\sqrt2$ et al are called "irrational". Commented Oct 14, 2011 at 3:11
• As I recall from reading a translation of The Geometry, Descartes actually talked about expressions involving "radicals of imaginary quantities", using "imaginary quantities" to refer to negative numbers, rather than to what we now call "imaginary numbers". So he would write a solution involving a square root, and say "if the quantity in the radical is imaginary, then ..." Commented Oct 14, 2011 at 3:17
• @J.M.: Exactly (also, he didn't restrict his axes to be perpendicular, but that's neither here nor there). The point is that, at the time, "imaginary" was an adjective used to describe negative numbers; saying Descartes coined the term "imaginary" in this context may be a bit misleading, I think. Commented Oct 14, 2011 at 3:27