In recent years this has been a very woolly term as defined in our Terms and Conditions:
2.3.4 you will not employ programs which consume excessive system resources, including but not limited to processor cycles and memory.
It came to our attention (after adding a number of new security applications recently) that we would only notify users when they were actually causing a problem that caused a service degradation to other users on the server. Now we have the new enhanced security we’re in a position where we will start emailing people if they exceed certain limits.
If you start receiving excessive resource usage emails on a regular basis you should work to reduce your usage to prevent further action being taken.