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.