Half-life
The time it takes for half of a radioactive substance, or anything decaying at a steady rate, to disappear.
A half-life is the time it takes for half of something to disappear when it decays at a steady proportional rate. It is most often used for radioactive atoms. Each unstable atom has a fixed chance of decaying in any given moment, so in one half-life about half the atoms in a sample decay, whatever the starting amount.
The process never simply finishes. After one half-life, half remains; after two, a quarter; after three, an eighth, or 12.5 per cent; after ten, about a thousandth. Carbon-14, used in radiocarbon dating, has a half-life of about 5,730 years. Other radioactive substances have half-lives of fractions of a second or of billions of years.
For a single atom, the half-life tells you only the odds, not when it will decay. It is like tossing coins: you cannot say which coin will land heads, but toss a million and very nearly half will. With the vast numbers of atoms in any real sample, the halving is extremely reliable, and that is what makes radioactive clocks work.
The idea is used outside physics too. The body clears many substances in the same pattern; the biological half-life of water in a human body is about 9 to 10 days, and the half-life of a medicine helps set how often it must be taken.
A common misconception is that after two half-lives everything is gone. A quarter is still there.