Sadness, n.
1. Heaviness; firmness. [Obs.]
2. Seriousness; gravity; discretion. [Obs.] Her sadness and her benignity. Chaucer.
3. Quality of being sad, or unhappy; gloominess; sorrowfulness; dejection. Dim sadness did not spare That time celestial visages. Milton.
Syn. -- Sorrow; heaviness; dejection. See Grief.
Shades of meaning
Fernald has no entry of its own for sadness; he lists it under grief.
Where he writes about it:
Sorrow and grief are for definite cause; sadness and melancholy may arise from a vague sense of want or loss, from a low state of health, or other ill-defined cause; sadness may be momentary; melancholy is more enduring, and may become chronic. — from grief