What does the half-life of a radioisotope correspond to?

I think half-life of a radioisotope corresponds to the time taken by on-half of the atoms of a radioactive substance to disintegrate or decay. Half life measures the rate at which a radioactive isotope decays. Radioisotopes have various half lives which may range from a few microseconds to billion of years. For example the half life of potassium-40 is 1.3 billion years.