A

acetaminophen:

Pronounced uh-SEE-tuh-MINN-uh fenn; a non-aspirin pain reliever, such as Tylenol.

acetylcholine:

Pronounced uh-settle-KOH-leen; a neurotransmitter that forms from a substance called choline, which is released by the liver.

acquired immunodeficiency syndrome (AIDS):

An infectious disease that destroys the body's immune system, leading to illness and death.

active ingredient:

The chemical or substance in a compound known or believed to have a therapeutic, or healing, effect.

adenosine triphosphate (ATP):

An important energy-carrying chemical, created with the assistance of creatine.

adrenaline:

Pronounced uh-DREN-uh-linn; a natural stimulant produced by the human body; also known as epinephrine (epp-ih-NEFF-run).

adverse reactions:

Side effects, or negative health consequences, reported after taking a certain substance.

aerobic exercises:

Exercises performed to increase heart health and stamina, such as jogging, biking, and swimming, usually lasting between twenty minutes and an hour.

aerosol:

Gas used to propel, or shoot out, liquid substances...