In the conflict of laws, lex causae (Latin: lex+causa, "cause law") is the law or laws chosen by the forum court from among the relevant legal systems to arrive at its judgement of an international or interjurisdictional case. The term refers to the usage of particular local laws as the basis or "cause" for the ruling, which would itself become part of referenced legal canon.
Conflict of laws regulates all lawsuits involving "foreign" law, where the outcome of a legal action will differ based on which laws are applied. Once the forum court has ruled that it has jurisdiction to hear the case, it must then decide which of the possible laws are to be applied.
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