The Collaborative International Dictionary of English v.0.48:

Lothario \Lo*tha"ri*o\, n. [Name of a character in Rowe's drama,
   "The Fair Penitent."]
   A gay seducer of women; a libertine.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Lothario
    n 1: a successful womanizer; a man who behaves selfishly in his
         sexual relationships with women

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "Lothario":
   Casanova, Don Juan, Romeo, amoroso, beau, boyfriend, caballero,
   cavalier, cavaliere servente, debauchee, esquire, fellow, flame,
   gallant, gay deceiver, gay dog, gigolo, inamorato, lady-killer,
   libertine, love-maker, lover-boy, man, necker, old man, petter,
   philanderer, profligate, rake, rakehell, rip, roue, rounder,
   seducer, sheik, skirt chaser, squire, sugar daddy, swain, swinger,
   walking phallus, wanton, wolf, woman chaser, womanizer,
   young man