Final answer:
The half-life of a radioactive substance is the time it takes for half of the original nuclei to decay or remain.
Step-by-step explanation:
The half-life of a radioactive substance is the time it takes for half of the original nuclei to decay or the time at which half of the original nuclei remain. For example, if the half-life of a substance is one hour, after one hour, only half of the original substance will remain. After another hour, half of the remaining substance will decay, and so on.