The legal doctrine of the binding force of precedent. Stare decisis is a feature of common-law legal systems (such as Australia) which operates to ensure consistency in the application of the law.
Stare decisis only applies to courts lower than the originating decision (e.g. a precedent from the High Court will be binding on the Supreme Court, but not the other way around). It is optional whether to defer to the precedent of the same Court (i.e. the High Court may either affirm or challenge its own past decisions) but in practice, it is very rare for courts to go against their own past rulings.