It Handles The Money
Or is critical to generating money not more than two removes from balance sheet.
I don't define most CRM as the above, but count CRM as a funnel. Reason: good sales bods used to do that in their head. So, for a sales app to be critical ("enterprise") you have to get to the bits an old school sales team can't do. It doesn't include lead search or credit analysis front of house. Critical in sales would be an engine which prices product in a way you secure sales over competitors. But that might be such a fluid piece of code, you'd never hard-wire it into any billing system.
If you're selling a service, that's obviously enterprise: no service, no money. But i think you were talking about internal apps here.
I dislike entrprise search / discovery being classified as entreprise. Archival matters. But you can always restore to a search server if you must, for legal e.g. later. If you need to use full text search to find line of business information, i think you've got a messy practise, to put it mildly. Yet, there sure is a lot of money in that space. My take is that such search utility is more a distraction, and at worst if used it ought to be rationed.
I started with my Razor, so i'll save typing any more expansions. I said 2 removes, to emphasise that in a enterprise sized company, if you go much further you have a very big graph to reduce as to what's important.
Seems everyone else here is going for Computer Contradictionary style answers. I'm trying to say that enterprise software is what you can't take away without losses. My org bonuses those in that trench in line with front of house / sales. Sometimes i think that enterprise software is deliberately arcane so that median salary employees can be "trusted" with apps near to the money because they'll have to be geniuses to know how anything works, and if they are, they'll leave. Whenever i get that feeling, a vendor gets shown the door.