Half-life

[Rennhack]

The time in which one half of the atoms of a radioactive isotope disintegrates into another nuclear form. Half-lives vary from billionths of a billionth of a second to billions of years. Also called physical or radiological half-life. 

Biological half-life - the time an organism takes to eliminate one half the amount of a compound or chemical on a strictly biological basis

Effective half life - incorporates both the radioactive and biological half-lives. It is used in calculating the dose received from an internal radiation source.

[Rennhack]

http://www.epa.gov/radiation/understand/halflife.htm