From cppreference.com
template<classR,classT,classU>conceptrelation=std::predicate<R,T,T>&&std::predicate<R,U,U>&&std::predicate<R,T,U>&&std::predicate<R,U,T>; (1) (since C++20)The concept relation<R, T, U> specifies that R defines a binary relation over the set of expressions whose type and value category are those encoded by either T or U.
References
C++23 standard (ISO/IEC 14882:2024):
18.7.5 Concept relation [concept.relation]
C++20 standard (ISO/IEC 14882:2020):
18.7.5 Concept relation [concept.relation]