Absolute radiometric dating definition
Note the consequence of the law of large numbers: with more atoms, the overall decay is more regular and more predictable.
A half-life usually describes the decay of discrete entities, such as radioactive atoms.
The decay of many physical quantities is not exponential—for example, the evaporation of water from a puddle, or (often) the chemical reaction of a molecule.
In such cases, the half-life is defined the same way as before: as the time elapsed before half of the original quantity has decayed.
In that case, it does not work to use the definition that states "half-life is the time required for exactly half of the entities to decay".
Circa is widely used in historical writing when the dates of events are not accurately known.
On the other hand, the time it will take a puddle to half-evaporate depends on how deep the puddle is.
Perhaps a puddle of a certain size will evaporate down to half its original volume in one day.
Simulation of many identical atoms undergoing radioactive decay, starting with either 4 atoms per box (left) or 400 (right).
The number at the top is how many half-lives have elapsed.