Steady state
Steady state is the point at which the amount entering the body over time and the amount being cleared have come into balance.
Before steady state, levels rise from one administration to the next. After it, they fluctuate around a stable average instead of climbing.
As a rule of thumb it takes roughly four to five half-lives to approach steady state, which is why something with a long half-life can keep accumulating for weeks.
What it is not
Steady state is not a plateau in effect, and reaching it is not evidence that anything is working. It is a statement about concentration over time and nothing else.
Where this comes up: half-life