Linguistics. a rule of transformational grammar that relates two phrase markers in the course of a derivation from the deep to the surface syntactic representation of a sentence, as by reordering, inserting, or deleting elements; a rule that converts deep structures into surface structures.
British Dictionary definitions for transformational rules
(generative grammar) a rule that converts one phrase marker into another. Taken together, these rules, which form the transformational component of the grammar, convert the deep structures of sentences into their surface structures
(pl) (logic) a rule that specifies in purely syntactic terms a method by which theorems may be derived from the axioms of a formal system