retinoic acid

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

• noun A vitamin A derivative, C₂₀H₂₈O₂, that is a regulator substance in morphogenesis and functions in the growth and development of bone and the maintenance of epithelium.
• noun One of two isomers of this chemical, used as a topical drug to treat acne and for cosmetic treatment of wrinkles and hyperpigmentation of facial skin.

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

• noun (Chem.) A retinoid compound (C20H28O2), closely related to vitamin A. It is also called tretinoin and vitamin A acid.

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

• noun A derivative of vitamin A, used in the treatment of acne; it is essential for bone and tissue development.