A figure of speech in which a word is repeated in a different form of the same root or stem, as Shakespeare's "then thou, whose shadow shadows doth make bright," or repeated with its word class changed into a different part of speech, as Tennyson's "my own heart's heart, and my ownest own, farewell."
The juxtaposition of common roots with different endings in a polyptoton produces a rhyme-like effect -- although not a true rhyme, it is sometimes referred to as a grammatical rhyme.
Sidelight: Similar to the polyptoton, but without involving repetition, is the anthimeria, frequently used by Shakespeare, which turns a word from one part of speech into another, usually in the making of verbs out of nouns, as in, "I'll unhair my head." Cummings boldly turned a verb and an adjective into nouns in the line, "they sowed their isn't they reaped their same."
(See also Antanaclasis, Epanalepsis, Epizeuxis, Ploce)

This definition is in context to Poetry. See more contextual defintions for Polyptoton (Puh-lip-tuh-tahn).