The Collaborative International Dictionary of English v.0.48:

Tercel \Ter"cel\, n.
   See Tiercel. Called also tarsel, tassel. --Chaucer.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tercel
    n 1: male hawk especially male peregrine or gyrfalcon [syn:
         tiercel, tercel, tercelet]