[...] "morpholexical rule" in the sense of Lieber [Lieber, R., 1980, “On the Organization of the Lexicon”, Doctoral dissertation, MIT, Cambridge]. That is, they are not phonological rules at all, but rather rules which express the relationships between allomorphs of morphemes, both of which are listed in the lexicon.
- Marantz (2004), Pag. 340

A morpholexical rule expresses the relation between two sets of allomorphs, a marked set and an unmarked set. (Lieber [Lieber, R., 1980, “On the Organization of the Lexicon”, Doctoral dissertation, MIT, Cambridge] calls the marked set "stems" and the unmarked set "roots" and provides an algorithm for picking out the root in a lexical entry). The marked set contains the "output" of the morpholexical rule wrongly cosidered as a phonological rule; the unmarked set contains the input.
- Marantz (2004), Pag. 350-351

Morpholexical rules are predicates which define sets of ordered pairs of lexical items [the "input" and "output" of the morpholexical rule], both of which are listed in the permanent lexicon.
- Marantz (2004), Pag. 346