Source : Webster's Revised Unabridged Dictionary (1913)

Stallion \Stal"lion\, n. [OE. stalon, OF. estalon, F. ['e]talon,
   fr. OHG. stal a stable. See {Stall}, n.]
   A male horse not castrated; a male horse kept for breeding.

Source : WordNet®

stallion
     n : uncastrated adult male horse [syn: {entire}]