A biological half-life or elimination half-life is the time it takes for a substance (drug, radioactive nuclide, or other) to lose one-half of its pharmacologic, physiologic, or radiological activity.In a medical context, the half-life may also describe the time that it takes for the concentration of a substance in blood plasma to reach one-half of its steady-state value (the "plasma half-life").(In other non-exponential decays, it can increase instead.) The decay of a mixture of two or more materials which each decay exponentially, but with different half-lives, is not exponential.

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The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo, or how long stable atoms survive, radioactive decay.

The term is also used more generally to characterize any type of exponential or non-exponential decay.

In a couple of minutes, almost all atoms of element A will have decayed after repeated halving of the initial number of atoms, but very few of the atoms of element B will have done so as only a tiny fraction of its half-life has elapsed.

Thus, the mixture taken as a whole will not decay by halves.

Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay on average".

In other words, the probability of a radioactive atom decaying within its half-life is 50%.

But on the second day, there is no reason to expect that one-quarter of the puddle will remain; in fact, it will probably be much less than that.

This is an example where the half-life reduces as time goes on.

In that case, it does not work to use the definition that states "half-life is the time required for exactly half of the entities to decay".