An online C++ compiler is a web-based tool that allows developers to write, compile, and execute C++ code without needing to install any software locally. These platforms provide a simple interface to input C++ code, run it in a cloud-based environment, and view the output instantly. Many online C++ compilers offer features like syntax highlighting, error checking, and debugging tools to enhance the coding experience. They are particularly useful for beginners learning C++, or for experienced developers needing to quickly test small code snippets without setting up an IDE. Popular examples include repl.it and ideone.com.