The Collaborative International Dictionary of English v.0.48:

Rodentia \Ro*den"ti*a\, n. pl. [NL. See Rodent, a.] (Zool.)
   An order of mammals having two (rarely four) large incisor
   teeth in each jaw, distant from the molar teeth. The rats,
   squirrels, rabbits, marmots, and beavers belong to this
   order.
   [1913 Webster]

   Note: The incisor teeth are long, curved, and strongly
         enameled on the outside, so as to keep a cutting edge.
         They have a persistent pulp and grow continuously.
         [1913 Webster]

WordNet (r) 3.0 (2006):

Rodentia
    n 1: small gnawing animals: porcupines; rats; mice; squirrels;
         marmots; beavers; gophers; voles; hamsters; guinea pigs;
         agoutis [syn: Rodentia, order Rodentia]