Synonyms for Sadness (same or very similar meaning)
WordNet sense 1 (the state of being sad):
sorrow, sorrowfulnessWordNet sense 2 (emotions experienced when not in a state of well-being):
unhappinessWordNet sense 3 (the quality of excessive mournfulness and uncheerfulness):
gloominess, lugubriousnessFrom the ODE community, based on WordNetadd/edit