# half-life

> English word · Noun · IPA /ˈhæfˌlaɪf/

## Definitions
1. The time required for half the nuclei in a sample of an isotope to undergo radioactive decay.
2. In a chemical reaction, the time required for the concentration of a reactant to fall from a chosen value to half that value.
3. The time it takes for a substance (drug, radioactive nuclide, or other) to lose half of its pharmacological, physiologic, or radiological activity.
4. The amount of time it takes for an idea or a fashion to lose half of its influential power.

## Etymology
From half- + life.

## Source
Compiled from Wiktionary via kaikki.org (CC BY-SA). Data vintage: 2026-05-06.
Canonical page: https://plainspell.com/en/word/half-life