In the context of United States law, originalism is a concept regarding the interpretation of the Constitution that asserts that all statements in the constitution must be interpreted based on the original understanding "at the time it was adopted". This concept views the Constitution as stable from the time of enactment and that the meaning of its contents can be changed only by the steps set out in Article Five. This notion stands in contrast to the concept of the Living Constitution, which asserts that the Constitution should be interpreted based on the context of current times and political identities, even if such interpretation is different from the original interpretations of the document. Living constitutionalists sometimes argue that we cannot apply an original understanding of the Constitution because the document is too old and too cryptic.

Proponents of originalism argue that originalism has historically been the primary method of legal interpretation in America from the time of its founding until the time of the New Deal, when competing theories of interpretation grew in prominence. Originalism was used by proponents of segregation to argue in opposition to civil rights legislation during the 1960s. Originalism is an umbrella term for interpretative methods that hold to the "fixation thesis", the notion that an utterance's semantic content is fixed at the time it is uttered. Originalists seek one of two alternative sources of meaning:

The original intent theory, which holds that interpretation of a written constitution is (or should be) consistent with what was meant by those who drafted and ratified it. This is currently a minority view among originalists. Alfred Avins and Raoul Berger (author of Government by Judiciary) are associated with this view.

The original meaning theory, which is closely related to textualism, is the view that interpretation of a written constitution or law should be based on what reasonable persons living at the time of its adoption would have understood the ordinary meaning of the text to be. Most originalists, such as Antonin Scalia, Clarence Thomas and Amy Coney Barrett, are associated with this view.

Such theories share the view that there is an identifiable original intent or original meaning, contemporaneous with the ratification of a constitution or statute, which should govern its subsequent interpretation. The divisions between the theories relate to what exactly that identifiable original intent or original meaning is: the intentions of the authors or the ratifiers, the original meaning of the text, a combination of the two, or the original meaning of the text but not its expected application.