The time, in a first-order chemical (or enzymic) reaction, for half of the substance (substrate) to be converted or to disappear. Cf.:half-life.
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(T1/2 , t1/2) (hafґtīm) the time required for one half of a quantity of a substance to be eliminated from a system when the substance is eliminated at a rate proportional to its concentration (i.e., exhibits first-order kinetics).Medical dictionary. 2011.