Res judicata means “a thing adjudicated”; “a case already decided”; or “a matter settled by a decision or judgment”.
Stare decisis means “to stand by decided cases”, “to uphold precedents”, “to maintain former adjudications”, or “not to disturb settled law”. Those things which have been so often adjudged ought to rest in peace.
Res judicata and Stare decisis are members of the same family. Both relate to adjudication of matters. Both deal with final determination of contested questions and have the binding effect in future litigation. Both the doctrines are the result of decisions of a competent court of law and based on public policy.
There is , however, distinction between the two. Whereas res judicata is based upon conclusiveness of judgement and adjudication of prior findings, stare decisis rests on legal principles.
Res judicata binds parties and privies, while stare decisis operates between stranger also and binds courts from taking a contrary view on the point of law already decided.
Res judicata relates to a specific controversy, stare decisis touches legal principles.
Res judicata presupposes judicial finding upon the same facts as involved in subsequent litigation between the same parties. Stare decisis applies to same principle of law to all parties.