It is usually used to describe quantities undergoing exponential decay (for example radioactive decay) where the half-life is constant over the whole life of the decay, and is a characteristic unit (a natural unit of scale) for the exponential decay equation.However, a half-life can also be defined for non-exponential decay processes, although in these cases the half-life varies throughout the decay process.
Students should be able to understand the principles and have that as a background so that age determinations by paleontologists and geologists don't seem like black magic. Geologists in the late 18th and early 19th century studied rock layers and the fossils in them to determine relative age.
William Smith was one of the most important scientists from this time who helped to develop knowledge of the succession of different fossils by studying their distribution through the sequence of sedimentary rocks in southern England.
Students not only want to know how old a fossil is, but they want to know how that age was determined.
Some very straightforward principles are used to determine the age of fossils.
The online Half Life Calculator is used to calculate the half-life in exponential decay.