Question: what is "Imaginary Number Space?" (spoilers)

I've seen this term used in two unrelated Japanese mecha series, both times used to describe essentially the same thing - a higher plane of existence that is home to unimaginably powerful entities. In the Xenosaga trilogy of video games, it is the dwelling place of "Upper Domain," the sentient energy field that inspired belief in God in humans. Meanwhile in Gurren Lagann, it's the home of the Anti-Spirals, described as being "between the 11th and 12th dimensions" and boasting such bizarre features as a giant robot-sized prison made of compacted universes and an infinitely deep and wide ocean floating through space.

I'm told something by this name also plays a major role in the Type-Moon/Nasuverse visual novels but I'm not familiar with those.

Anybody know where this term/concept originates and why multiple unrelated works would use it? I thought it might be some trippy thing from quantum physics or string theory or some such in real-world science but I haven't found anything in that regard.

Thanks.