Answer:
Half-life is defined as the time required by an unstable element to reduce half of its original value. All unstable elements have a decay rate and this half-life differs from element to element. These unstable elements are known as radioactive elements. It continuously decays until it becomes a stable one. These elements when decays release radiations which are harmful to lives.
This half-life plays an important role in the determination of the age of a rock or fossils. It is used in various types of radiometric dating methods.
For example, the half-life of Carbon-14 is approximately 5700 years and the half-life of Uranium-238 is 4.5 billion years.