Time to bite the bullet, I guess. What do we mean by 'enterprise'?
Consulting the Oracle, more formally known as Wikipedia, we find, in the Enterprise software entry:
Enterprise application software is application software that performs business functions such as accounting, production scheduling, customer information tracking, bank account maintenance, and the like. It is almost always hosted on servers, and is used by multiple employees of the same organisation. It can also be any software application hosted on a server which simultaneously provides services to a large number of users, typically over a computer network. This definition contrasts the more common single-user software applications which run on the user's own local computer, and serve only one user at a time.
Actually, y'know? That's not bad. The first half is the 'traditional' definition of 'enterprise', perhaps, but I want to zoom in on the bolded sentence: "It can be any software application hosted on a server which simultaneously provides services to a large number of users, typically over a computer network.". The main reason I'm aiming for that is that it then covers not only internal CMS's, publishing applications and the like, but also end-user facing systems (such as, for example, Amazon or Flickr).
Let's take that second sentence as our prototype definition, and dissect it to see what it implies. I'm very aware that I'm reaching, just a touch, with some of this, but it's a very good jumping off point to allow us to reach a definition of 'enterprise'.