Physically, the constant represents the time it takes the system's step response to reach of its final (asymptotic) value for systems that increase in value (say from a step increase), or it can represent the time for systems to decrease in value to (say from a step decrease). In radioactive decay the time constant is called the decay constant (λ), and it represents both the mean lifetime of a decaying system (such as an atom) before it decays, or the time it takes for all but 36.8% of the atoms to decay. For this reason, the time constant is longer than the half-life, which is the time for only 50% of the atoms to decay.