A runtime environment is a collection of software and hardware resources that provide the necessary infrastructure for the execution of programs. It includes the operating system, libraries, frameworks, and other dependencies that an application needs to run. The runtime environment manages the execution of the program, including memory allocation, input/output operations, and error handling. It ensures that the program behaves consistently across different platforms and configurations by abstracting the underlying hardware and system details. Examples of runtime environments include the Java Runtime Environment (JRE) for Java applications and the .NET Runtime for .NET applications.