Half-life is the time taken by a substance (molecule, drug or other) to lose half of its proportion. Used by extension in the field of radioactivity, the half-life, also called radioactive half-life, is the time at which half of the radioactive nuclei of a source have disintegrated.
The term half-life is often misinterpreted: two half-lives do not correspond to the full life of the product. The half-life is, in fact, the median of the life of a product, that is to say, the duration below which there remains more than 50% of the product, and beyond which there remains less than 50%. So two half-lives correspond to the duration where 50% of the 50% of the product remains, ie 25% (the quarter).