The Collaborative International Dictionary of English v.0.48:

Sweetheart \Sweet"heart`\, n.
   A lover of mistress.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sweetheart
    adj 1: privileged treatment of a favored person or corporation
           (sometimes unethically); "another sweetheart deal based
           on political influence"
    n 1: a person loved by another person [syn: sweetheart,
         sweetie, steady, truelove]
    2: any well-liked individual; "he's a sweetheart"
    3: a very attractive or seductive looking woman [syn: smasher,
       stunner, knockout, beauty, ravisher, sweetheart,
       peach, lulu, looker, mantrap, dish]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "sweetheart":
   ace, admirer, angel, babe, baby, baby-doll, beau, beaut, beloved,
   betrothed, boyfriend, buttercup, captive, catch, cherub, chick,
   chickabiddy, conquest, coquette, corker, court, crackerjack, daisy,
   dandy, darb, darling, date, dear, deary, dilly, doll, dream,
   dreamboat, duck, duckling, fiancee, flame, flirt, friend,
   heartthrob, hon, honey, honey bunch, honey child, humdinger,
   inamorata, inamorato, intended, killer-diller, knockout, ladylove,
   lamb, lambkin, lollapaloosa, love, lover, lulu, make up to,
   paramour, peach, pet, petkins, pip, pippin, precious,
   precious heart, pursue, snookums, spark, steady, sue, sugar, sweet,
   sweet patootie, sweetie, sweetkins, sweets, the nuts, truelove,
   turtledove, vamp, vampire, whiz, woo