What is Free Warren?

Legal Definition
Free warren—often simply warren—refers to a type of franchise or privilege conveyed by a sovereign in mediaeval England to a subject, promising to hold them harmless for killing game of certain species within a stipulated area, usually a wood or small forest. The sovereign involved might be either the monarch or a marcher lord.
-- Wikipedia
Legal Definition
Eng. law. A franchise erected for the preservation and custody of beasts and fowls of warren. 2 Bl. Com. 39; Co. Litt. 233.
-- Bouviers Law Dictionary
Legal Definition
A franchise for the preserving and custody of beasts and fowls of warren. 2 Bl. Comm. 39, 417; Co. Litt. 233. This franchise gave the grantee sole right of killing, so far as his warren extended, on condition of excluding other persons. 2 Bl. Comm. 39.
-- Black's Law Dictionary
Legal Definition
A franchise of hunting and preserving game in a warren.
-- Ballentine's Law Dictionary