It is Latin for to stand by things decided, and it is the reason the law does not change every time a new judge takes the bench. Until, sometimes, dramatically, it does.
Stare decisis is the doctrine that courts should follow their own past decisions, called precedents, when ruling on similar new cases. The full Latin phrase is stare decisis et non quieta movere: to stand by decisions and not disturb settled matters.
Its purpose is stability and fairness. If the law shifted with every judge's mood, no one could plan their affairs or trust the courts. Stare decisis makes the law predictable, treats like cases alike, and constrains judges from simply imposing their personal views.
It is strong but not iron. Lower courts must follow the precedents of higher courts above them. But a high court can, in rare and weighty cases, overrule its own past decisions, as the Supreme Court did in Brown v. Board of Education, overturning the separate but equal doctrine.
Those reversals are seismic precisely because they are rare. When the highest court overturns a long-standing precedent, it signals that the earlier ruling was so wrong, or the world so changed, that stability must yield. Such moments reshape the country.
