A half-life is the amount of time required for half of a given substance to decay. Half-life is applied to materials that experience exponential decay, meaning that the rate of decay is directly proportional to the amount of the substance, slowing down as the substance depletes. Half-life varies widely — it can be a few seconds or millions of years, depending upon the stability of the substance. The concept of half-life has applications throughout the world of science.

Half-life is always the same for a given substance, until it reaches very small amounts. Though the rate of decay slows over time, the half-life remains constant. Imagine an element has a half-life of five minutes, for example. If we start out with 20 ounces of it, five minutes later we will have 10 ounces remaining, and in another five minutes only 5 ounces remaining. The rate of decay has slowed from two ounces per minute to one ounce per minute, but the half-life is constant at five minutes.

Half-life is perhaps best known in the context of radioactivity. Radioactive "parent" elements transform into stable, or non-radioactive, "daughter" elements and emit radiation as they decay. Knowing the daughter element and half-life of a radioactive element, therefore, allows one to date a partially decayed sample by comparing the ratio of parent to daughter element. This method of dating is commonly used to determine the age of various entities in our universe, from fossils to meteorites. Carbon-14, a radioactive element present in living materials on Earth, begins decaying at the moment of death and can be used to date once-living materials such as bone and wood.

Half-life is also important in biology, where it indicates the amount of time in which half of a given substance is metabolized and/or eliminated by the body. In this sense, half-life is used in pharmacology to determine appropriate dosage amounts and intervals. It can also be applied in toxicology to determine the effects of toxins in the body over time.