| − | The time it takes for half of the original sample of a radioactive isotope to decay.<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref> | + | The time it takes for half of the original sample of a [[radioactive]] [[isotope]] to decay.<ref>Wile, Dr. Jay L. ''Exploring Creation With Physical Science''. Apologia Educational Ministries, Inc. 1999, 2000</ref> |