A unicorn is a company that has reached 1bn valuation while staying private. So Microsoft, Oracle, Google and Amazon don't qualify (while Google definitely was one for some time, it's just that the term didn't exist back then). Of course it also puts PwC and Bechtel in this league...
How is it that Avast is still a "startup" when it was founded in 1988?
I mean.. they were founded even before the dot com bubble!
Edit: I know there's always a problem when trying to define what is and isn't a startup...but by all metrics I could think, Avast isn't one of them. I mean, otherwise we would also need to include the likes of Microsoft, Oracle, Google, Amazon as other unicorn startups, no?