Half-life is a measurement of the time it takes for one-half of a radioactive substance to decay (in this sense, decay does not mean to rot, but to diminish in quantity).
The atoms of radioactive substances, such as uranium and radium, spontaneously break down over time, transforming themselves into atoms of another element. In the process, they give off radiation, or energy emitted in the form of waves. An important feature of the radioactive decay process is that each substance decays at its own rate. The half-life of a particular substance, therefore, is constant and is not affected by any physical conditions (temperature, pressure, etc.) that occur around it.
Because of this stable process, scientists are able to estimate when a particular substance was formed by measuring the amount of original and transformed atoms in that substance. For example, the amount of carbon in a fossil sample can be measured to determine the age of that fossil. It is known that the radioactive substance carbon-14 has a half-life of 5,570 years. The half-lives of other radioactive substances can range from tiny fractions of a second to quadrillions of years.