Weird, I found the concept so obvious I didn't notice the lack of a definition. An 'onramp' is a wooly concept because so many things could constitute one. A killer use case, a flagship company using the tech and shouting about it, maybe even just a great book/documentation. I think, and again this is without consulting someone else's definition, you need many, diverse, onramps, to become popular.

I distinctly remember being introduced to Go, it was around launch time but there was already a working compiler suite (that was easy to install from source on OSX and Linux), website, tutorial and introductory talks. Rather than Google backing per se, you could just say it was a well-launched product, but the two are probably intertwined.