Source : Webster's Revised Unabridged Dictionary (1913)

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

            The fevor of ensuing day.             --Waller.

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

            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.

Source : WordNet®

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}]
     2: feelings of great warmth and intensity; "he spoke with great
        ardor" [syn: {ardor}, {ardour}, {fervor}, {fervency}, {fire},
         {fervidness}]