1.  sadness(n = noun.feeling) unhappiness - emotions experienced when not in a state of well-being;
is a kind of feeling
has particulars: dolefulness, heaviness, melancholy, misery, desolation, forlornness, loneliness, tearfulness, weepiness, sorrow, regret, rue, ruefulness, sorrow, cheerlessness, uncheerfulness, depression, dejectedness, dispiritedness, downheartedness, low-spiritedness, lowness
Antonym: happiness
Derived form adjective sad₁

2.  sadness(n = noun.state) sorrow, sorrowfulness - the state of being sad; "she tired of his perpetual sadness"
is a kind of unhappiness
has particulars: bereavement, mourning, poignance, poignancy
Derived form adjective sad₁

3.  sadness(n = noun.attribute) gloominess, lugubriousness - the quality of excessive mournfulness and uncheerfulness;
is a kind of uncheerfulness
Derived forms adjective sad₃, adjective sad₂

1.  Heaviness; firmness. [1913 Webster]

2.  Seriousness; gravity; discretion. [1913 Webster]
"Her sadness and her benignity." [1913 Webster]

3.  Quality of being sad, or unhappy; gloominess; sorrowfulness; dejection. [1913 Webster]
"Dim sadness did not spare
That time celestial visages." [1913 Webster]

N  dejection, dejectedness, depression, prosternation, lowness of spirits, depression of spirits, weight on the spirits, oppression on the spirits, damp on the spirits, low spirits, bad spirits, drooping spirits, depressed spirits, heart sinking, heaviness of heart, failure of heart, heaviness, infestivity, gloom, weariness, taedium vitae, disgust of life, mal du pays, anhedonia, melancholy, sadness, il penseroso, melancholia, dismals, blues, lachrymals, mumps, dumps, blue devils, doldrums, vapors, megrims, spleen, horrors, hypochondriasis, pessimism, la maladie sans maladie, despondency, slough of Despond, disconsolateness, hope deferred, blank despondency, voiceless woe, prostration of soul, broken heart, despair, cave of despair, cave of Trophonius demureness, gravity, solemnity, long face, grave face, hypochondriac, seek sorrow, self-tormentor, heautontimorumenos, malade imaginaire, medecin tant pis, croaker, pessimist, mope, mopus, affliction, sorry sight, memento mori, damper, wet blanket, Job's comforter, cheerless, joyless, spiritless, uncheerful, uncheery, unlively, unhappy, melancholy, dismal, somber, dark, gloomy, triste, clouded, murky, lowering, frowning, lugubrious, funereal, mournful, lamentable, dreadful, dreary, flat, dull, dull as a beetle, dull as ditchwater, depressing, melancholy as a gib cat, oppressed with melancholy, a prey to melancholy, downcast, downhearted, down in the mouth, down in one's luck, heavy-hearted, in the dumps, down in the dumps, in the suds, in the sulks, in the doldrums, in doleful dumps, in bad humor, sullen, mumpish, dumpish, mopish, moping, moody, glum, sulky, out of sorts, out of humor, out of heart, out of spirits, ill at ease, low spirited, in low spirits, a cup too low, weary, discouraged, disheartened, desponding, chapfallen, chopfallen, jaw fallen, crest fallen, sad, pensive, penseroso, tristful, dolesome, doleful, woebegone, lacrymose, lachrymose, in tears, melancholic, hypped, hypochondriacal, bilious, jaundiced, atrabilious, saturnine, splenetic, lackadaisical, serious, sedate, staid, stayed, grave as a judge, grave as an undertaker, grave as a mustard pot, sober, sober as a judge, solemn, demure, grim, grim-faced, grim-visaged, rueful, wan, long-faced, disconsolate, unconsolable, inconsolable, forlorn, comfortless, desolate, desole, sick at heart, soul sick, heart sick, au desespoir, in despair, lost, overcome, broken down, borne down, bowed down, heartstricken, cut up, dashed, sunk, unnerved, unmanned, down fallen, downtrodden, broken-hearted, careworn, with a long face, with tears in one's eyes, sadly, the countenance falling, the heart failing, the heart sinking within one, a plague of sighing and grief, thick-ey'd musing and curs'd melancholy, the sickening pang of hope deferred.