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Definition of Half-life

Babylon English

futuristic computer adventure game manufactured by Valve Software
amount of time it takes for half of the atoms in a radioactive substance to break down or decay
Half-life Definition from Business & Finance Dictionaries & Glossaries
Campbell R. Harvey's Hypertextual Finance Glossary
The point in the life of a mortgage backed security  guaranteed or issued by the Government National Mortgage Association, the Federal National Mortgage Association or the Federal Home Loan Mortgage Corporation when half the principle has been repaid.
Copyright © 2000, Campbell R. Harvey. All Rights Reserved.
Glossary of petroleum Industry
the amount of time needed for half of a quantity of radioactive substance to decay or transmute into a nonradioactive substance. Half-lives range from fractions of seconds to millions of years.
Half-life Definition from Government Dictionaries & Glossaries
DOD Dictionary of Military Terms
(*) The time required for the activity of a given radioactive species to decrease to half of its initial value due to radioactive decay. The half-life is a characteristic property of each radioactive species and is independent of its amount or condition. The effective half-life of a given isotope is the time in which the quantity in the body will decrease to half as a result of both radioactive decay and biological elimination.
  
Source: U.S. Department of Defense, Joint Doctrine Division. ( About )
Half-life Definition from Science & Technology Dictionaries & Glossaries
General Chemistry Glossary
The half life of a reaction is the time required for the amount of reactant to drop to one half its initial value.
NRC Nuclear Energy Glossary
The time in which one half of the atoms of a particular radioactive substance disintegrate into another nuclear form. Measured half-lives vary from millionths of a second to billions of years. Also called physical or radiological half-life.
The time required for the body to eliminate one half of the material taken in by natural biological means.
The time required for a radionuclide contained in a biological system, such as a human or an animal, to reduce its activity by one-half as a combined result of radioactive decay and biological elimination.
A service to the public by The U.S. Nuclear Regulatory Commission (NRC).
EIA Energy Glossary
The time it takes for an isotope to lose half of its radioactivity.
Source: Energy Information Administration, 2006
Technical English by wpv
The time in which half the atoms of a radioactive substance will have disintegrated, leaving half the original amount. Half of the residue will disintegrate in another equal period of time.
ASTRONOMY UNBOUND
The time taken for half the atoms in a radioactive substance to decay. Each radio-nuclide has a unique half-life, they extend from (say) 10-40 second to infinite life. The positive and negative electron and proton, together with the photon, are stable. However there are important consequences to astrophysical theories if the proton is unstable but with an exceedingly long mean lifetime.
Half-life Definition from Computer & Internet Dictionaries & Glossaries
Electronic Games
Half-Life, 1st person shooter.

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Half-life Definition from Encyclopedia Dictionaries & Glossaries
English Wikipedia - The Free Encyclopedia
Half-life (t1/2) is the amount of time required for the amount of something to fall to half its initial value. The term is very commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay, but it is also used more generally for discussing any type of exponential decay.

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Half-life is a mathematical and scientific description of exponential or gradual decay.

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© This article uses material from Wikipedia® and is licensed under the GNU Free Documentation License and under the Creative Commons Attribution-ShareAlike License
Half-life Definition from Society & Culture Dictionaries & Glossaries
EPA Terms of Environment
1. The time required for a pollutant to lose one-half of its original coconcentrationor example, the biochemical half-life of DDT in the environment is 15 years. 2. The time required for half of the atoms of a radioactive element to undergo self-transmutation or decay (half-life of radium is 1620 years). 3. The time required for the elimination of half a total dose from the body.
Provided as a public service by the U.S. Environmental Protection Agency
Environmental Engineering (English ver.)
The time required for a pollutant to lose one-half of its original concentraton.
Half-life Definition from Entertainment & Music Dictionaries & Glossaries
English to Federation-Standard Golic Vulcan
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Half-life Definition from Medicine Dictionaries & Glossaries
The TCRC Glossary for Testicular Cancer and Other Related Conditons
The time required for half the amount of a substance (such as a tumor marker) in or introduced into the body to be eliminated or disintegrated by natural processes.
CDC Radiological Terms
 the time any substance takes to decay by half of its original amount. See also biological half-life, decay constant, effective half-life, radioactive half-life.
  
Source: Centers for Disease Control and Prevention (CDC) ( About )
TOXICOLOGY
The time required for a concentration of a substance in a body fluid (usually blood plasma) to decrease by half.
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Muscle Pain
time for half of a substance to be converted or disappear from the tissue.
Terms and symbols used in pharmacology
The period of time required for the concentration or amount of drug in the body to be reduced to exactly one-half of a given concentration or amount. The given concentration or amount need not be the maximum observed during the course of the experiment, or the concentration or amount present at the beginning of an experiment, since the half-life is completely independent of the concentration or amount chosen as the "starting point". Half-lives can be computed and interpreted legitimately only when concentration or amount varies with time according to the law appropriate to the kinetics of a first order reaction: the common logarithm of the concentration or amount is related linearly to time, e.g.:
log C=a+bt
where C is concentration at time t, a (in logarithmic units) is the intercept of the line with the ordinate, and b (which has a negative sign) is the slope of the line. The parameters of the equation can be estimated from the plot of experimental values of log C and t. The half-life can be computed simply by dividing the slope of the curve into 0.301, the difference between the logarithm of a number (C) and the logarithm of number half as large (C/2); the symbol for half-life is t1/2.
The half-life of a drug in plasma or serum is frequently taken as indicating the persistence of the drug in its volume of distribution; this interpretation may be incorrect unless the material can move freely and rapidly from one fluid compartment of the body to another, and is not bound or stored in one or another tissue. The term "biological half-life" should not be used instead of the specific terms "plasma half-life" or "serum half-life". The tissue for which the half-life of a drug is determined should always be specified, e.g., "serum half-life"; the half-life of a drug in muscle, kidney, etc., or in the whole organism can be determined. Drug half-lives are frequently based on the results of chemical analyses, i.e., the results of the reaction of a reagent with a specific chemical group of a drug molecule; it should be remembered that detection of the group per se does not necessarily imply its continuous existence as part of a biologically active drug molecule.

A drug molecule that leaves the plasma may have any of several fates: it can be destroyed in the blood; it can be eliminated from the body; or it can be translocated to a body fluid compartment other
than the intravascular to be stored, biotransformed, or to exert its pharmacodynamic effects.

When the plot of log plasma or serum concentration (during the period of its decline) against time is composed of two straight line segments, the inference may be made that two first order processes are involved in the distribution and biotransformation and elimination of the drug. The earlier phase - represented by the line segment of greater slope - is termed the distributive phase, and corresponds to
the period during which translocation of the drug to its ultimate volume of distribution occurs and is the dominant process; the later phase - represented by the line of lesser slope - is termed the eliminative phase, and corresponds to the period when biotransformation and elimination of drug are dominant processes. For two-phase systems, three phase systems, etc., half-lives of the drugs in the various phases can be determined only after more sophisticated analysis of the data than that described above.

Cf. First Order Kinetics, Compartment, Volume of Distribution, Pharmacokinetics, Biotransformation, Biotranslocation, a, b
Aids Glossary
the time required for half the amount of an agent (e.g., drug, virus, cell type) to be eliminated from the body.
Aegis
Glossary of HIV/AIDS-Related Terms
The time required for half the amount of a drug to be eliminated from the body.
ATIS
HIV Vaccine Glossary
the time required for half the amount of a substance to be eliminated from the body or to be converted to another substance(s).
Half-life Definition from Language, Idioms & Slang Dictionaries & Glossaries
WordNet 2.0

Noun
1. the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)
(synonym) half-life
(hypernym) time period, period of time, period

Noun
1. the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)
(synonym) half life
(hypernym) time period, period of time, period