The Collaborative International Dictionary of English v.0.48:

Quercus \Quer"cus\, n. [L., an oak.] (Bot.)
   A genus of trees constituted by the oak. See Oak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Quercus
    n 1: oaks [syn: Quercus, genus Quercus]