Posterior Scalp: Back of the head.

Preauricular Flap: Surgical procedure that takes hair-bearing scalp from the temple area and rotates it about 90 degrees to the front, forming a hairline.

Progesterone: Female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. Synthetic compounds with progesterone-like activity have been developed that, often along with estrogen, are used in oral contraceptives.

Propecia: The brand name for 1mg dose of finasteride, approved for the prevention and treatment of male pattern baldness.

Prosthetic: An artificial replacement.

Punch Graft: A group of ten to twenty hairs in a circular graft.

Recipient Site: Bald area where hair grafts are transplanted.

Rejection: Tissue not accepted by the body and which, therefore, dies.

Retin-A: A brand name for a prescription acne medication. Has in some cases shown to be effective against hair loss, particularly when combined with minoxidil. It can cause extreme scalp irritation that can make hair loss worse.

Retroauricular Area: Area behind the ear.

Rogaine: The brand name for minoxidil topical hair growth solution, available over the counter in 2% solution and in a 5% extra strength solution.

Rotational Flap: A surgical procedure that lifts a three-sided area of hair-bearing scalp and pivots it 90 to 180 degrees into the balding area.

Scalp Reduction: Surgical procedure in which an ellipse of bald scalp is removed from a small midline bald spot and the hair-bearing scalp between the ears is pulled together and sutured closed. This reduces the bald area.

Scarring Alopecia: Patchy hair loss with obvious sign of scalp inflammation.

Scleroderma: A disease of the skin and connective tissue that can cause hair loss over the affected areas.

Sebaceous Glands: Fatty glands found in hair follicles throughout the body that secrete an oil into the hair and surrounding skin.

Seborrheic Dermatitis: A condition marked by oily, scaly patches or spots on the skin. It frequently occurs on the face and scalp.

Sebum: An oily secretion manufactured by tiny sebaceous glands near the follicles that keeps hair lubricated and shiny.

Senescent Alopecia: The type of hair loss that naturally occurs with age, when both the duration of hair growth and the diameter of the hair follicle decrease.

Shock Fallout: The condition that occurs when hair transplantation is performed on men with a significant amount of naturally occurring hair left on their head. Trauma due to the procedure itself induces a telogen phase for much of the hair around the implanted grafts. Hair lost due to shock fallout returns in some cases.

Slit Graft: A graft of three to four hairs inserted into a slit rather than a round hole.

SOD: Also known as Superoxide Dismutase, enzymes that destroy superoxide free radicals and prevent the cellular damage that free radicals cause. Researchers have discovered that SODases also stimulate hair growth and decrease hair loss.