Younger branches of noble families are called cadets, because
their armorial shields are marked with a difference called a cadency.
is a student at the Royal Military Academy at Woolwich, the Royal
Military College at Sandhurst, or in one of her Majesty's training
ships, the Excellent
and the Britannia.
From these places
they are sent (after passing certain examinations) into the army as
ensigns or second lieutenants, and into the navy as midshipmen.
junior member of a family.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Cadet from Fact Monster: