The Collaborative International Dictionary of English v.0.48:

Portent \Por*tent"\ (?; 277), n. [L. portentum. See Portend.]
   That which portends, or foretoken; esp., that which portends
   evil; a sign of coming calamity; an omen; a sign. --Shak.
   [1913 Webster]

         My loss by dire portents the god foretold. --Dryden.
   [1913 Webster]

WordNet (r) 3.0 (2006):

portent
    n 1: a sign of something about to happen; "he looked for an omen
         before going into battle" [syn: omen, portent,
         presage, prognostic, prognostication, prodigy]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "portent":
   adumbration, advance notice, augury, auspice, balefulness,
   banefulness, betokening, betokenment, bodefulness, boding,
   direness, doomfulness, fatality, fatefulness, foreboding,
   foreshadow, foreshadowing, foreshowing, foretoken, foretokening,
   forewarning, indicant, indication, marvel, meaning, miracle, omen,
   ominousness, phenomenon, plenty of notice, portentousness,
   precautioning, prefiguration, preindication, premonition,
   premonitory shiver, premonitory sign, premonitory symptom,
   prenotice, prenotification, presage, presagefulness, presentiment,
   presignifying, prewarning, prodigy, prognostic, prognostication,
   promise, sensation, shadow, sign, significance, sinisterness,
   soothsay, stunner, suggestiveness, token, tokening, type