Definition
A runtime is the environment and supporting system in which a program actually executes: memory management, scheduling, standard libraries, and interaction with the operating system. The word also refers to the concrete software hosting a language’s execution, such as Node.js or a browser’s JavaScript engine.
How it works
Once a program starts, the runtime loads the code, allocates memory, executes it and reclaims resources; garbage-collected languages delegate memory management to the runtime. The runtime exposes capabilities to the program through built-in APIs, such as reading files or making network requests.
Usage
The same JavaScript code can manipulate a page inside a browser runtime or read and write files inside a Node.js runtime — which built-in capabilities a program can use depends on the runtime. Java programs, meanwhile, run on the JVM.
Related and confusable terms
An API is the calling convention a runtime exposes to programs; the runtime is the environment that executes those calls. Runtime is contrasted with compile time: the former is the phase of execution, the latter the phase of translating source code into machine code.