The Collaborative International Dictionary of English v.0.48:

Ardor \Ar"dor\, n. [L. ardor, fr. ardere to burn: cf. OF. ardor,
   ardur, F. ardeur.] [Spelt also ardour.]
   1. Heat, in a literal sense; as, the ardor of the sun's rays.
      [1913 Webster]

   2. Warmth or heat of passion or affection; eagerness; zeal;
      as, he pursues study with ardor; the fought with ardor;
      martial ardor.
      [1913 Webster]

   3. pl. Bright and effulgent spirits; seraphim. [Thus used by
      Milton.]
      [1913 Webster]

   Syn: Fervor; warmth; eagerness. See Fervor.
        [1913 Webster]

WordNet (r) 3.0 (2006):

ardor
    n 1: a feeling of strong eagerness (usually in favor of a person
         or cause); "they were imbued with a revolutionary ardor";
         "he felt a kind of religious zeal" [syn: ardor, ardour,
         elan, zeal]
    2: intense feeling of love [syn: ardor, ardour]
    3: feelings of great warmth and intensity; "he spoke with great
       ardor" [syn: ardor, ardour, fervor, fervour,
       fervency, fire, fervidness]

Moby Thesaurus II by Grady Ward, 1.0:

198 Moby Thesaurus words for "ardor":
   Amor, Christian love, Eros, Platonic love, abandon, acquiescence,
   activity, admiration, adoration, affection, agape, agreeability,
   agreeableness, alacrity, allegiance, amenability, anger, animation,
   application, ardency, assiduity, assiduousness, attachment,
   avidity, bodily love, brio, briskness, brotherly love, calenture,
   caritas, charity, cheerful consent, commitment, committedness,
   compliance, concentration, concupiscence, conjugal love, consent,
   cooperativeness, curiosity, dedication, desideration, desire,
   devotedness, devotion, devoutness, diligence, docility, drive,
   eagerness, earnestness, ecstasy, elan, energeticalness, energy,
   enthusiasm, excitement, faith, faithful love, faithfulness, fancy,
   fantasy, favorable disposition, favorableness, fealty, fervency,
   fervidity, fervidness, fervor, fidelity, fieriness, fire, flame,
   fondness, forwardness, free love, free-lovism, furor, fury,
   galvanization, gameness, glow, goodwill, gusto, heart, heartiness,
   heat, heatedness, hero worship, hope, horme, hurrah, idolatry,
   idolism, idolization, impassionedness, impetuosity, impetus,
   indefatigability, industriousness, industry,
   intellectual curiosity, intensity, intentness, joie de vivre,
   keenness, laboriousness, lasciviousness, libido, life, like,
   liking, liveliness, love, lovemaking, loyalty, lust for learning,
   lustiness, married love, mettle, mind, need, passion,
   passionateness, perkiness, pertness, physical love, piety,
   pleasure, pleasure principle, pliability, pliancy, popular regard,
   popularity, promptness, quickening, readiness, receptive mood,
   receptiveness, receptivity, regard, relentlessness, relish,
   resolution, responsiveness, right mood, robustness, savor,
   sedulity, sedulousness, sentiment, seriousness, sex, sexual desire,
   sexual love, shine, sincerity, soul, spirit, spiritedness,
   spiritual love, stimulation, strenuousness, tender feeling,
   tender passion, thirst for knowledge, tirelessness, tractability,
   truelove, ungrudgingness, unloathness, unreluctance, unsparingness,
   urge, uxoriousness, vehemence, verve, vivacity, want, wanting,
   warmth, warmth of feeling, weakness, will, will and pleasure,
   willing ear, willing heart, willingness, wish, wish fulfillment,
   worship, yearning, zeal, zealousness, zest, zestfulness, zing



The Devil's Dictionary (1881-1906):

ARDOR, n.  The quality that distinguishes love without knowledge.