A precise definition of the isotopic composition of water on Earth. It serves as a reference standard for comparing hydrogen and oxygen isotope ratios. For example, cosmochemists measure the ¹⁷O / ¹⁶O and ¹⁸O / ¹⁶O ratios in terms of deviations in parts per thousand from a standard (delta ¹⁸O and delta ¹⁷O). The usual standard is SMOW.