5 Quantities and units library [quantities]

5.4 Reference [qty.ref]

5.4.4 Unit [qty.unit]

5.4.4.5 Types [qty.unit.types]

5.4.4.5.7 One [qty.unit.one]

namespace mp_units { struct one final : derived-unit-impl<> {}; }
one represents the base unit (IEC 60050, 112-01-18) of a quantity of dimension one (IEC 60050, 112-01-13).