Question

What do you understand by the half-life a reaction.

Answer 1

The amount of time it takes for half an initial amount to disintegrate.

The time required for half of a reactant to be converted into product(s). The time required for half of a given sample to undergo radioactive decay. Definition for  Half-Life :The time it takes for one-half of the atoms of an unstable element or nuclide to decay radioactively into another element or nuclide

For a given reaction the half life t1/2 of a reactant is the time required for its concentration to reach a value that is the arithmetic mean of its initial and final (equilibrium) value. For a reactant that is entirely consumed it is the time taken for the reactant concentration to fall to one half of its initial value. For a first-order reaction, the half life of the reactant may be called the half life of the reaction. In nuclear chemistry, (radioactive) half life is defined, for a simple radioactive decay process, as the time required for the activity to decrease to half its value by that process. See also biological half life.

Answer 2

Hey Buddy.

Here is your answer.
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The half-life of a reaction is the time required for the reactant concentration to decrease to one-half its initial value. The half-life of a first-order reaction is a constant that is related to the rate constant for the reaction: t1/2 = 0.693/k. Radioactive decay reactions are first-order reactions.

Hope it's help for you.