(1):

(n.) A change in the form or function of a living organism, by a natural process of growth or development; as, the metamorphosis of the yolk into the embryo, of a tadpole into a frog, or of a bud into a blossom. Especially, that form of sexual reproduction in which an embryo undergoes a series of marked changes of external form, as the chrysalis stage, pupa stage, etc., in insects. In these intermediate stages sexual reproduction is usually impossible, but they ultimately pass into final and sexually developed forms, from the union of which organisms are produced which pass through the same cycle of changes. See Transformation.

(2):

(n.) The change of material of one kind into another through the agency of the living organism; metabolism.

(3):

(n.) Change of form, or structure; transformation.