What is halved
Half-life concerns the remaining parent nuclei of a particular radioactive species. After one interval half remain on average; after two, one quarter remain. This does not mean that the entire sample’s mass halves repeatedly, because decay products can remain while the sample’s nuclear composition changes.
A regular law for random events
An individual nucleus has no scheduled decay appointment determined by its age. Exponential decay describes large populations through a probability per unit time under specified conditions. A larger decay constant corresponds to a shorter half-life. Mean lifetime is related to half-life but is a different quantity.
References: [1]
Mixtures and decay chains
A sample may contain several radioactive species, and daughter nuclei may also be radioactive. The total detector signal therefore need not follow one simple half-life. Analysis must identify the nuclide and radiation being measured and consider whether material enters the system or daughters accumulate.
References: [2]
Conditions for dating
Dating with radioactive decay also requires information about initial composition, exchange history and calibration. Activity is not itself an age or a dose received by a person. Half-life provides a time dependence, which becomes useful only when combined with a suitable model of the sample.