What does an isotope’s half-life measure?

In the context of chemistry and radioactivity, an isotope’s half-life measures the specific interval of time required for one-half of the atomic nuclei in a radioactive sample to undergo radioactive decay and transform into other nuclear species.12

Equivalently, it represents the time needed for the number of disintegrations per second of a radioactive material to decrease by half. Because each radioisotope decays at a constant, intrinsic rate, the half-life is a characteristic property of that specific unstable nucleus.132

The half-life is inversely proportional to the decay constant ($\lambda$), which represents the isotope's intrinsic rapidity of decay. This relationship is expressed by the formula:32
$$T_{1/2}=\frac{0.693}{\lambda }$$
where $T_{1/2}$ is the half-life. This measurement is fundamental to fields such as geochronology, where it allows scientists to calculate the age of materials by measuring the ratio of parent isotopes to their decay products.23