The time required for a quantity of a radioisotope to decay by half. For example, because the half-life of iodine-131 (i-131) is 8 days, a sample of i-131 that has 10 mci of activity on january 1, will have 5 mci of activity 8 days later, on january 9. See also biological half-life, decay constant, effective half-life. (illustrations)