One who plunders or pillages without the authority of national warfare; a member of a predatory band; a pillager; a buccaneer; a sea robber.

This is just another name for a pirate.

someone who takes spoils or plunder (as in war)

A conqueror in a small way of business, whose annexations lack of the sanctifying merit of magnitude.

Freebooter is a fictional character who appeared in the Marvel Comics's series A-Next.