A runtime system refers to the collection of software and hardware resources that enable a software program to be executed on a computer system. The runtime system is a composite mechanism designed to provide program execution services, regardless of the programming language being used.
Techopedia explains Runtime System
A runtime system is primarily created by the programming language in the way it presents the programming architecture that's native to the program. Typically, a runtime system provides both low level and high level commands, interfacing with both the primary software framework/library and the underlying hardware instructions set architecture (ISA). The low-level functions provided by a runtime system include processor interfacing, memory loading and digital-to-binary conversion, among others. The higher level functions include type checking, code generation, and debugging and optimization services.
Java Runtime Environment, for example, is a type of runtime system that provides the complete framework for executing and managing Java programs.