rejuvenescence

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A renewal of youthful appearance or character.

from The Century Dictionary.

• noun A renewal of the appearance, powers, or feelings of youth.
• noun In biology, a transformation whereby the entire protoplasm of a vegetative cell changes into a cell of a different character—that is, into a primordial cell which subsequently invests itself with a new cell-wall and forms the starting-point of the life of a new individual. It occurs in numerous algæ, as Œdogonium, and also in some diatoms.
• noun The renewal of vitality, which has been exhausted through repeated cell-division, by the sexual union of two cells into a cell of compound origin.

from the GNU version of the Collaborative International Dictionary of English.

• noun A renewing of youth; the state of being or growing young again.
• noun (Bot.) A method of cell formation in which the entire protoplasm of an old cell escapes by rupture of the cell wall, and then develops a new cell wall. It is seen sometimes in the formation of zoöspores, etc.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A renewal of youthful characteristics or vitality