The Collaborative International Dictionary of English v.0.48:

Fervor \Fer"vor\, n. [Written also fervour.] [OF. fervor,
   fervour, F. ferveur, L. fervor, fr. fervere. See Fervent.]
   1. Heat; excessive warmth.
      [1913 Webster]

            The fevor of ensuing day.             --Waller.
      [1913 Webster]

   2. Intensity of feeling or expression; glowing ardor;
      passion; holy zeal; earnestness. --Hooker.
      [1913 Webster]

            Winged with fervor of her love.       --Shak.

   Syn: Fervor, Ardor.

   Usage: Fervor is a boiling heat, and ardor is a burning heat.
          Hence, in metaphor, we commonly use fervor and its
          derivatives when we conceive of thoughts or emotions
          under the image of ebullition, or as pouring
          themselves forth. Thus we speak of the fervor of
          passion, fervid declamation, fervid importunity,
          fervent supplication, fervent desires, etc. Ardent is
          used when we think of anything as springing from a
          deepseated glow of soul; as, ardent friendship, ardent
          zeal, ardent devotedness; burning with ardor for the
          fight.
          [1913 Webster]

WordNet (r) 3.0 (2006):

fervour
    n 1: the state of being emotionally aroused and worked up; "his
         face was flushed with excitement and his hands trembled";
         "he tried to calm those who were in a state of extreme
         inflammation" [syn: excitement, excitation,
         inflammation, fervor, fervour]
    2: feelings of great warmth and intensity; "he spoke with great
       ardor" [syn: ardor, ardour, fervor, fervour,
       fervency, fire, fervidness]