This term was first used in 1907 as "half-life period" but in our short attention span society, it was shortened to just "half-life." Either way, it has three definitions, according to the Environmental Protection Agency (EPA): 1. The time required for a pollutant to lose one-half of its original coconcentrationor example. 2. The time required for half of the atoms of a radioactive element to undergo self-transmutation or decay. 3. The time required for the elimination of half a total dose from the body.
The Nuclear Information and Resource Service (NIRS) says, "Reactor waste remains hazardous for a very long time. Most medical waste from treatment and diagnosis is hazardous for a very short time. Research and industrial waste can contain small amounts of some long-lived radioactive materials."
Some examples: The half-life of DDT in the environment is 15 years; strontium-90 has a half-life of 28 years; nickel-59 has a half-life of 76,000 years; the half-life of radium is 1620 years; for uranium-235 it's 704 million years; and the half-life of uranium-238 is about 4.47 billion years. Thus, you can understand why half-life usually refers to all thingsnuclear.
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