A runtime system, also called run-time system, primarily implements portions of an execution model. This is in contrast to the runtime lifecycle phase of a program, during which the runtime system is in operation. Most languages have some form of runtime system, which implements control over the order in which work that was specified in terms of the language gets performed. Over the years, the meaning of the term 'runtime system' has been expanded to include nearly any behaviors that are dynamically determined during execution.