General Science


  • The time taken for half of the atoms of an unstable isotope to decay. Half-lives can be under a second (0.3 seconds for Sodium 20) or millions of years (about 4.5 billion years for Uranium 238).


  • noun the number of years needed to repay half the capital borrowed on mortgage


  • The period of time that the level of radioactivity for a substance decays by 50%.


  • The time interval that elapses for a given radioactive substance to lose half of its radioactive intensity. That is, it is the time required for half the atoms of a given sample of identical nuclides to undergo decay. The half-life of 50V (an isotope of vanadium), for instance, is approximately 1.4 × 1017 years, while that for 6Be (an isotope of beryllium) is approximately 5.9 × 10-21 seconds. Also called radioactive half-life.


  • noun the time taken for half the cells in a tissue sample to die and be replaced


  • noun a measurement of the period of time taken before the concentration of a drug has reached half of what it was when it was administered
  • noun the time taken for half the atoms in a radioactive isotope to decay