Intensity is the amount of your power you have to use to complete a repetition of an exercise. The highest intensity possible is often called the “one rep max” (or “1RM”) – a weight you can handle just once when doing the movement of an exercise. At the 1RM you are using 100% of your available power output.
Via: http://evilcyber.com/fitness/bodybuilding-intensity-volume/#ixzz3nP99f3XI