Since working with speed-power athletes I've always defined intensity as a % of what a person is capable of relative to their 100% maximal. So in weightlifting terms, I'm basically speaking of activities in the 85+% range or activities performed with maximal speed. One way I like to differentiate / reconcile the various different definitions of intensity by making a clear cut distinction between effort and intensity. Most coaches and athletes understand effort regardless of their background (endurance vs. speed power) or sport. As a general guideline:
*High intensity activity = very high effort over a very short period of time.
*Medium intensity activity = medium to high effort over a short to medium duration period of time.
*Low intensity activity = low to medium effort over a short to medium duration of time.