A company whose revenue is primarily derived from a technology characterized by high margin, high scalability and low fixed costs. One could also say that such a technology should be the primary differentiator for the product. The products therefore tend towards software and hardware as applied to a pre-existing market, but that's not an ironclad rule.

Know this mostly isn't looking for an answer, but I think HN typically means something along the lines of:

"one with a significant development arm of their employees/business that is crucial to the company's success"

So any company run off an app or website that's complex and well made is probably a "tech company" even if it's in the business of cars, e-commerce, or widgets. There's no hard and fast rule here of course, but we don't need to pretend we don't know the gist of what people actually mean, even if "tech company" is a misnomer in many ways.

A company where software engineering is considered part of R&D and not IT.

If every human stopped working at the company for a day the product would still work (not a meatspace product). The company will typically have a high GAAP gross margin.

I think for the purposes of this question, "IPO whose S-1 gets posted to HN" is a good definition. No need to overthink it.

It's "the companies that we like". Simple. /s