The half-life of a radioactive substance is the length of time during which the activity of the substance reduces by 50%. In other words, it is the time required for half of the initial number of radioactive atoms to decay. This means that after one half-life, half of the initial amount of the radioactive substance will have decayed, and after two half-lives, one quarter of the initial amount will remain. The concept of half-life is important in radioactivity, as it can be used to predict the decay rate and the amount of time it will take for a given amount of the substance to decay.