Yeah, it definitely isn't a standardized term. I've also been hearing "hard tech" but then got a feeling it has more and more been used for hardware projects, although I dont think that that was the original idea.

We actually never insisted on being called deep-tech, but we realised that was how others characterized us, especially when talking to investors (building a new programming language sounds pretty scary, although in our case it is a non turing complete DSL).