The Collaborative International Dictionary of English v.0.48:

Torch song \Torch" song`\, n.
   a popular song with a sentimental theme of unrequited love or
   other form of unhappiness.
   [PJC]

WordNet (r) 3.0 (2006):

torch song
    n 1: a popular song concerned with disappointment in love

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "torch song":
   Brautlied, Christmas carol, Kunstlied, Liebeslied, Volkslied, alba,
   anthem, art song, aubade, ballad, ballade, ballata, barcarole,
   blues, blues song, boat song, bridal hymn, brindisi, calypso,
   canso, canticle, canzone, canzonet, canzonetta, carol, cavatina,
   chanson, chant, chantey, croon, croon song, dirge, ditty,
   drinking song, epithalamium, folk song, hymeneal, lay, lied, lilt,
   love song, love-lilt, matin, minstrel song, minstrelsy,
   national anthem, prothalamium, serena, serenade, serenata, song,
   theme song, war song, wedding song