Stare decusus is a Latin term meaning "to stand by that which is decided."
In short it is the principal that the precedent decisions are to be followed by the courts. Courts cite stare decisis when an issue has been previously brought to the court and a ruling already issued. Generally, courts will adhere to the previous ruling, though this is not universally true.
It is a general maxim that when a point has been settled by decision, it forms a precedent which is not afterwards to be departed from. However one case, Bush v. Gore the Supreme Court explicitly stated that it could never be used as a precedent, since it would invalidate every past and future election.
The doctrine of stare decisis, while a general guide to the court, is not always to be strictly relied upon, as the courts may find it necessary to overrule cases which have been hastily decided, or contrary to principle. Many hundreds of such overruled cases may be found in the American and English books of reports.
Although the doctrine of stare decisis does not prevent reexamining and, if need be, overruling prior decisions, it is based on the assumption that certainty, predictability and stability in the law are the major objectives of the legal system. In other words, parties should be able to regulate their conduct and enter into relationships with reasonable assurance of the governing rules of law. Accordingly, a party urging overruling a precedent faces a rightly onerous task, the difficulty of which is roughly proportional to a number of factors, including the age of the precedent, the nature and extent of public and private reliance on it, and its consistency or inconsistency with other related rules of law.