Definition
How heavy the load is relative to a one-rep max, not how hard the session feels.
In strength coaching, intensity means load as a percentage of 1RM. In gym talk it means effort. Keep the two apart in your own notes: a set of 20 at 50% is high effort and low intensity.
Rough zones: 60 to 70% for sets of 8 to 12, 75 to 85% for sets of 4 to 6, 85 to 95% for sets of 1 to 3. Intensity and volume trade off; a block cannot maximise both, which is most of what periodization is about.
Track it as the average intensity of the working sets each week. Rising average intensity at a steady volume means the client is getting stronger, whatever the scale says.