Intensity, n.
Etymology: [LL. intensitas: cf. F. intensité. See Intense.]
1. The state or quality of being intense; intenseness; extreme degree; as, intensity of heat, cold, mental application, passion, etc. If you would deepen the intensity of light, you must be content to bring into deeper blackness and more distinct and definite outline the shade that accompanies it. F. W. Robertson.
2. (Physics)
The amount or degree of energy with which a force operates or a cause acts; effectiveness, as estimated by results produced.
3. (Mech.)
3 more senses
The magnitude of a distributed force, as pressure, stress, weight, etc., per unit of surface, or of volume, as the case may be; as, the measure of the intensity of a total stress of forty pounds which is distributed uniformly over a surface of four square inches area is ten pounds per square inch.
4. (Photog.)
The degree or depth of shade in a picture.
Shades of meaning
Fernald has no entry of its own for intensity; he lists it under enthusiasm.
Where he writes about it:
Against the hindrances of the world, nothing great and good can be carried without a certain fervor, intensity, and vehemence; these joined with faith, courage, and hopefulness make enthusiasm. — from enthusiasm