The decay rate, or activity, of a radioactive substance is characterized by the following time-independent parameters:
* The half-life, , is the time taken for the activity of a given amount of a radioactive substance to decay to half of its initial value.
* The decay constant,  "lambda", the reciprocal of the mean lifetime (in ), sometimes referred to as simply decay rate.
* The mean lifetime,  "tau", the average lifetime (1/e life) of a radioactive particle before decay.
Although these are constants, they are associated with the statistical behavior of populations of atoms. In consequence, predictions using these constants are less accurate for minuscule samples of atoms.
In principle a half-life, a third-life, or even a (1/√2)-life, can be used in exactly the same way as half-life; but the mean life and half-life  have been adopted as standard times associated with exponential decay.
Those parameters can be related to the following time-dependent parameters:
* Total activity (or just activity), , is the number of decays per unit time of a radioactive sample.
* Number of particles, , in the sample.
* Specific activity, , is the number of decays per unit time per amount of substance of the sample at time set to zero (). "Amount of substance" can be the mass, volume or moles of the initial sample.
These are related as follows:
where N is the initial amount of active substance — substance that has the same percentage of unstable particles as when the substance was formed.