Original: The name Britain is derived from the Latin name Britannia (earlier Brittannia), via Old French Bretaigne (whence also Modern French Bretagne) and Middle English Bretayne, Breteyne.
Simplified: The name Britain comes from the Latin word Britannia. In Old French the word was Bretaigne which became Bretagne in Modern French and Bretayne, Breteyne in Middle English.