Customization point object (since C++20)

From cppreference.com

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
Memory management library
Metaprogramming library (C++11)
General utilities library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library (C++17)
Concurrency support library (C++11)
Execution control library (C++26)
Technical specifications
Symbols index
External libraries

Customization point object (C++20)
Exposition-only entities
decay-copy (C++11)
synth-three-waysynth-three-way-result (C++20)(C++20)

The name cpo denotes a customization point object, which is a const function object of a literal semiregular class type. See CustomizationPointObject for details.