The Planck constant, , is one of seven fixed constants defining the International System of Units, the SI, with . The Planck constant is considered as a fundamental constant of nature.
Sources: BIPM, The International System of Units, SI Brochure, 9th ed. (2019), p. 127ff [Terms] [Book] Newell et al., Metrologia 55, L13-L16 (2018) [Terms] [Source]