Half-life
A half-life is the time it takes for the amount of a substance in the body to fall by half.
It is a property of a molecule in a given population, measured in studies, and it is usually quoted as an average with real variation around it.
Half-lives matter because they describe accumulation. When something is taken again before the previous amount has substantially cleared, the level in the body builds up over successive administrations until intake and clearance balance.
For example
If a substance has a 24-hour half-life, then 24 hours after it enters the body roughly half remains, after 48 hours roughly a quarter, and after 72 hours roughly an eighth.
What it is not
A half-life is not how long an effect lasts. Effects can outlast measurable levels or fade well before them, and the two are separate questions.
Where this comes up: half-life