Thanks for mentioning @PamCotton 
Hey @JQuick0 thanks for popping this up, already made the request for this beta!
Anyway, regarding the definition here you go: “The last time a contact engaged with your site or a form, document, meetings link, or tracked email. This doesn’t include marketing emails or emails to multiple contacts.” so things you mentioned like “Multiple page views in single session; Downloads PDFs from website; Replies to any email; Registers for a webinar” will be there but not focus only on that.
What’s my idea (and I already make something similar to alert real agents) you create a property called high intent date where you set a new date through workflow everytime a contact or company (in my case I’m doing account based) checks your own criteria.
If we’re able to “call” the prospecting agent by custom workflow it’s quite easy - you simply need to deactivate the workflow on your print and set a new one that will be triggered by you new property called high intent date, if we can’t do that, since we’re not able to update “last engagement” property, I’ll need to wait to get the beta to dig into that.