Half-life Definition & Meaning

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half-life American [haf-lahyf] / ˈhæfˌlaɪf / Or half life,

noun

plural

half-lives

1. Physics. the time required for one half the atoms of a given amount of a radioactive substance to disintegrate.

2. Also called biological half-life. Pharmacology. the time required for the activity of a substance taken into the body to lose one half its initial effectiveness.

3. Informal. a brief period during which something flourishes before dying out.

half-life British

noun

1. τ. the time taken for half of the atoms in a radioactive material to undergo decay

2. 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 )

"Collins English Dictionary — Complete & Unabridged" 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 half-life Scientific / hăf′līf′ /

1. 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.

half-life Cultural

1. 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.

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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.)

Etymology

Origin of half-life

First recorded in 1905–10; half + life

Example Sentences

Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.

So why, in this glum post-“Walking Dead” era, awaken our much-loved caveman from a good death to such a miserable half-life?

From Salon

“When I talk to a truck driver, they say this is great. They’re not talking about its half-life.”

From The Wall Street Journal

Deuterium is abundant, but tritium is scarce because it is radioactive, with a half-life of only 12.3 years.

From The Wall Street Journal

At-211's short half-life also means it quickly loses its radioactivity, making it less toxic than longer-lived radiopharmaceuticals.

From Science Daily

If an isotope is said to have a half-life of five years, you can expect roughly half of the atoms to have decayed in that amount of time.

From Literature

Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.