Source : Webster's Revised Unabridged Dictionary (1913)

Monotreme \Mon"o*treme\, n. [Cf. F. monotr[`e]me.] (Zo["o]l.)
   One of the Monotremata.

Source : WordNet®

monotreme
     n : the most primitive mammals comprising the only extant
         members of the subclass Prototheria [syn: {egg-laying
         mammal}]