From PHPEverywhere today:

Jeff Moore talks about his struggles trying to define enterprise computing. Like Jeff, I don't have a precise answer. But I have a couple of litmus tests that you can apply:

• Is there an MIS department involved?
• There are a large number of users - it needs to be several hundred or more.
• Are users in several geographical locations?
• Is it mission-critical - does operational failure of 1 day mean that someone very important will be screaming at MIS?
• Cost of project - did someone spend hundreds of thousands of US dollars or more?
• Is there a central server (or multiple servers) and data processing in the background?
• Is the value derived from the project (return on investment) by the stakeholders a large amount (eg. a couple of million dollars a year)?

This suggests that only the largest of projects can be considered "enterprise", but I can think of a few situations where, despite the fact that a limited number of users access it, the application is very much key to how a company operates...