What were reduction sentences in Rudolf Carnap’s account of scientific language?

In Rudolf Carnap’s philosophy, reduction sentences are logically refined versions of operational definitions used to link the terms of empirical science to observable phenomena.1

As Carnap moved away from his earlier, stricter requirement that all empirical terms be fully definable through immediate experience, he introduced reduction sentences in his essay "Testability and Meaning" (1936–37) to provide a more liberal account of empirical significance. These sentences allow theoretical terms to be only partially defined by connecting them to observation sentences—statements whose truth can be checked through direct observation, such as reading a measuring instrument.1

By utilizing these sentences, Carnap argued that scientific statements could be tested and receive "confirmation" from observational findings, even if they could not be strictly proven or disproven. This framework was central to his broader project of physicalism, which sought to show that all terms in empirical science could be reduced to the language of physics.1