The half-life of a radioactive substance is the time it takes for half of the radioactive atoms in the substance to decay. It is a measure of the rate at which a radioactive substance decays. The half-life of a radioactive substance is a constant, meaning it does not change over time.

For example, if a radioactive substance has a half-life of 1 hour, then after 1 hour, half of the radioactive atoms in the substance will have decayed. After another 1 hour, half of the remaining radioactive atoms will have decayed, and so on.

The half-life of a radioactive substance is used to determine how long it will take for the substance to become safe to handle. For example, if a radioactive substance has a half-life of 100 years, then it will take 100 years for the substance to decay to a level that is safe to handle.