Full Idea
We say something prescriptive if and only if, for some act A, some situation S and some person R, if P were to assent (orally) to what we say, and not, in S, do A, he logically must be assenting insincerely.
Gist of Idea
A judgement is presciptive if we expect it to be acted on
Source
Richard M. Hare (Moral Thinking: Its Levels,Method and Point [1981], p.21), quoted by Philippa Foot - Does Moral Subjectivism Rest on a Mistake? p.190
Book Reference
Foot,Philippa: 'Moral Dilemmas' [OUP 2002], p.190
A Reaction
Foot offers this as Hare's most explicit definition. The use of algebra strikes me as ludicrous. In logic letters have the virtue of not shifting their meaning during an argument, but that is not required here.