The Collaborative International Dictionary of English v.0.48:

Myomorpha \My`o*mor"pha\, n. pl. [NL., fr. Gr. my^s, myo`s, a
   mouse + morfh` form.] (Zool.)
   An extensive group of rodents which includes the rats, mice,
   jerboas, and many allied forms.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Myomorpha
    n 1: true rats and mice and related rodents [syn: Myomorpha,
         suborder Myomorpha]