A person’s idea of how his or her body looks.

An eating disorder characterized by self-starvation. Affected individuals voluntarily undereat and often overexercise, depriving their vital organs of nutrition. Anorexia can be fatal.

The long-term upward or downward direction of a certain set of statistical measurements, as opposed to a smaller, shorter cyclical variation. As an example, over the last two centuries, because of improved nutrition and medical care, children have tended to reach their adult height earlier and their adult height has increased.

The parts of the body that are directly involved in reproduction, including the vagina, uterus, ovaries, testicles, and penis.

A sex hormone, considered the chief estrogen. Females produce much more estradiol than males do.

The relatively sudden and rapid physical growth that occurs during puberty. Each body part increases in size on a schedule: Weight usually precedes height, and growth of the limbs precedes growth of the torso.

Two glands, located above the kidneys, that produce hormones (including the “stress hormones” epinephrine [adrenaline] and norepinephrine).

A sequence of hormone production originating in the hypothalamus and moving to the pituitary and then to the adrenal glands.

An organic chemical substance that is produced by one body tissue and conveyed via the bloodstream to another to affect some physiological function.

The paired sex glands (ovaries in females, testicles in males). The gonads produce hormones and gametes.

A sex hormone, the best known of the androgens (male hormones); secreted in far greater amounts by males than by females.

Physical traits that are not directly involved in reproduction but that indicate sexual maturity, such as a man’s beard and a woman’s breasts.

A hormone that affects appetite and is believed to affect the onset of puberty. Leptin levels increase during childhood and peak at around age 12.

A girl’s first menstrual period, signaling that she has begun ovulation. Pregnancy is biologically possible, but ovulation and menstruation are often irregular for years after menarche.

A day–night cycle of biological activity that occurs approximately every 24 hours (circadian means “about a day”).

A boy’s first ejaculation of sperm. Erections can occur as early as infancy, but ejaculation signals sperm production. Spermarche may occur during sleep (in a “wet dream”) or via direct stimulation.

A sequence of hormone production originating in the hypothalamus and moving to the pituitary and then to the gonads.

A gland in the brain that responds to a signal from the hypothalamus by producing many hormones, including those that regulate growth and that control other glands, among them the adrenal and sex glands.

The time between the first onrush of hormones and full adult physical development. Puberty usually lasts three to five years. Many more years are required to achieve psychosocial maturity.

Any erotic activity that arouses an adult and excites, shames, or confuses a child, whether or not the victim protests and whether or not genital contact is involved.

A disease spread by sexual contact, including syphilis, gonorrhea, genital herpes, chlamydia, and HIV.

An eating disorder characterized by binge eating and subsequent purging, usually by induced vomiting and/or use of laxatives.