[haf-lahyf, hahf-] /ˈhæfˌlaɪf, ˈhɑf-/
noun, plural half-lives
[haf-lahyvz, hahf-] /ˈhæfˌlaɪvz, ˈhɑf-/ (Show IPA)
Physics. the time required for one the atoms of a given amount of a radioactive substance to disintegrate.
Also called biological half-life. Pharmacology. the time required for the activity of a substance taken into the body to lose one its initial effectiveness.
Informal. a brief period during which something flourishes before dying out.
the time taken for half of the atoms in a radioactive material to undergo decay τ
the time required for half of a quantity of radioactive material absorbed by a living tissue or organism to be naturally eliminated (biological half-life) or removed by both elimination and decay (effective half-life)
also halflife, half life, 1864, with meaning “unsatisfactory way of living;” the sense in physics, “amount of time it takes half a given amount of radioactivity to decay” is first attested 1907.
The average time needed for half the nuclei in a sample of a radioactive substance to undergo radioactive decay. The half-life of a substance does not equal half of its full duration of radioactivity. For example, if one starts with 100 grams of radium 229, whose half-life is 4 minutes, then after 4 minutes only 50 grams of radium will be left in the sample, after 8 minutes 25 grams will be left, after 12 minutes 12.5 grams will be left, and so on.
In physics, a fixed time required for half the radioactive nuclei in a substance to decay. Half-lives of radioactive substances can range from fractions of a second to billions of years, and they are always the same for a given nucleus, regardless of temperature or other conditions. If an object contains a pound of a radioactive substance with a half-life of fifty years, at the end of that time there will be half a pound of the radioactive substance left undecayed in the object. After another fifty years, a quarter-pound will be left undecayed, and so on.
Note: Scientists can estimate the age of an object, such as a rock, by carefully measuring the amounts of decayed and undecayed nuclei in the object. Comparing that to the half-life of the nuclei tells when they started to decay and, therefore, how old the object is. (See radioactive dating.)
- Half bill
noun phrase Fifty dollars: Half an hour after I’d lent Reno the half bill, he was back uptown (1920s+)
[haf-lahyt, hahf-] /ˈhæfˌlaɪt, ˈhɑf-/ noun 1. light that is about half its customary brightness, or that is partially dimmed or obscured: the half-light of early dawn; a room in half-light. noun 1. a dim light, as at dawn or dusk
[haf-beek, hahf-] /ˈhæfˌbik, ˈhɑf-/ noun 1. any of several marine fishes of the family Hemiramphidae, having a greatly elongated lower jaw. /ˈhɑːfˌbiːk/ noun 1. any marine and freshwater teleost fish of the tropical and subtropical family Hemiramphidae, having an elongated body with a short upper jaw and a long protruding lower jaw
noun 1. a bathroom containing only a toilet and wash basin; powder room.