A distinct and separate production environment with its own collection of database tables in the database. More information.
Solutions must be deployed to the runtime system before they can be used in production.
Different definition, for when multiple runtime systems are supported:
A distinct and separate production environment with its own collection of database tables that is found on one or more database servers. If you are using only one runtime system for DOCUMENTS, it is referred to as a "built-in" runtime system. [Maybe link to "Creating a runtime system" or something]