1.  loneliness(n = noun.state) solitariness - the state of being alone in solitary isolation;
is a kind of isolation
Derived form adjective lonely₁

2.  loneliness(n = noun.feeling) desolation, forlornness - sadness resulting from being forsaken or abandoned;
is a kind of sadness, unhappiness
Derived form adjective lonely₂

3.  loneliness(n = noun.attribute) aloneness, lonesomeness, solitariness - a disposition toward being alone;
is a kind of disposition, temperament
has particulars: friendlessness, reclusiveness
Derived form adjective lonely₃

1.  The condition of being lonely; solitude; seclusion. [1913 Webster]

2.  The state of being unfrequented by human beings; as, the loneliness of a road. [1913 Webster]

3.  Love of retirement; disposition to solitude. [1913 Webster]
"I see
The mystery of your loneliness." [1913 Webster]

4.  A feeling of depression resulting from being alone. [1913 Webster]

N  exclusion, nonadmission, omission, exception, rejection, repudiation, exile, noninclusion, preclusion, prohibition, separation, segregation, seposition, elimination, expulsion, cofferdam, excluding, exclusive, excluded, unrecounted, not included in, inadmissible, exclusive of, barring, except, with the exception of, save, bating, exclusion, seclusion, exclusion, seclusion, privacy, retirement, reclusion, recess, snugness, delitescence, rustication, rus in urbe, solitude, solitariness, isolation, loneliness, estrangement from the world, voluntary exile, aloofness, cell, hermitage, convent, sanctum sanctorum, depopulation, desertion, desolation, wilderness, howling wilderness, rotten borough, Old Sarum, exclusion, excommunication, banishment, exile, ostracism, proscription, cut, cut direct, dead cut, inhospitality, inhospitableness, dissociability, domesticity, Darby and Joan, recluse, hermit, eremite, cenobite, anchoret, anchorite, Simon Stylites, troglodyte, Timon of Athens, Santon, solitaire, ruralist, disciple of Zimmermann, closet cynic, Diogenes, outcast, Pariah, castaway, pilgarlic, wastrel, foundling, wilding, secluded, sequestered, retired, delitescent, private, bye, out of the world, out of the way, the world forgetting by the world forgot, snug, domestic, stay-at-home, unsociable, unsocial, dissocial, inhospitable, cynical, inconversable, unclubbable, sauvage, troglodytic, solitary, lonely, lonesome, isolated, single, estranged, unfrequented, uninhabitable, uninhabited, tenantless, abandoned, deserted, deserted in one's utmost need, unfriended, kithless, friendless, homeless, lorn, forlorn, desolate, unvisited, unintroduced, uninvited, unwelcome, under a cloud, left to shift for oneself, derelict, outcast, banished, noli me tangere, among them but not of them, and homeless near a thousand homes I stood, far from the madding crowd's ignoble strife, makes a solitude and calls it peace, magna civitas magna solitudo, never less alone than when alone, O sacred solitude! divine retreat!.