Assumpsit (“he has undertaken”, from Latin, assumere), or more fully, action in assumpsit, was a form of action at common law used to enforce what are now called obligations arising in tort and contract; and in some common law jurisdictions, unjust enrichment.

"Assumpsit" is a Latin term used in law that means "he has undertaken." It refers to a type of action or lawsuit where the plaintiff asserts that the defendant has not fulfilled a promise or obligation that was not officially contracted or agreed upon. This could be a verbal agreement or an implied promise. The term is most commonly used in the context of contract law.