[TOP]half-life 'half-life noun [C] 1.(physics 物) the time taken for the radioactivity of a substance to fall to half its original value (放射性物质的)半衰期 2.(chemistry 化) the time taken for the concentration of a reactant to fall to half of its original value in a chemical reaction 半反应期 [TOP]half-life 'half-life noun [C] 1. the length of time it takes a RADIOACTIVE substance to lose half of its RADIOACTIVITY 〔放射性物质的〕半衰期 [TOP]half-lifeUK ['hɑ:f.laɪf] US ['hæf-] noun countable specialized » the length of time needed for the radioactivity of a radioactive substance to be reduced by half （放射性物质的）半衰期 [TOP]half-life
1.N-COUNT 可数名词 （放射性物质的）半衰期 The half-life of a radioactive substance is the amount of time that it takes to lose half its radioactivity.