prescriptive

a. [ L. praescriptivus of a demurrer or legal exception. ] 1. (Law) Consisting in, or acquired by, immemorial or long-continued use and enjoyment; as, a prescriptive right of title; pleading the continuance and authority of long custom. [ 1913 Webster ]

The right to be drowsy in protracted toil has become prescriptive. J. M. Mason. [ 1913 Webster ]

2. Of or pertaining to the doctrine that acceptable grammatical rules should be prescribed by authority, rather than be determined by common usage. [ PJC ]