Originalism is a method of constitutional interpretation that focuses on how a provision of a constitution would have been understood at the time of its ratification.
The most common form is so-called "original meaning" originalism. This form that focuses on how ordinary people at the time would have understood the language of the constitutional provision. A largely-discarded form of orginalism is so-called "original intent" originalism, which focuses on what the authors of the constitution might have meant.
The philosophical basis of originalism is that a constitution only has force because it was approved by the people when it was ratified. Thus, the understanding of the constitution by the people who ratified it is the only valid interpretation.
Originalists reject the "evolving standards of decency" approach to constitutional interpretation that allows judges to effectively amend the constitution based on their own views of what the constitution "should" say. Instead, originalism is anchored in one certain interpretation.
Originalism is closely associated with Justice Antonin Scalia, Justice Clarence Thomas, and scholar Robert Bork, all of whom have publicly supported its use and produced scholarship in support of it.