Which definition describes a half-life

For example, the half-life of a radioactive substance is the amount of time it takes for half of its atoms

Answer: Half life is the time in which a substance remains halve of its original amount. Mostly we use the term half life for radioactive substances.

For example: - Half life of carbon-14 is 5730 years. So, if we take 100 grams of carbon-14 then it will remain 50 grams in 5730 years. In next 5730 years, it will remain 25 grams.