Software engine

A software engine is a core component of a complex software system. The word "engine" is a metaphor of a car's engine. Thus a software engine is a complex subsystem.
There is no formal guideline for what should be called an engine, but the term has become entrenched in the software industry. Notable examples are database engine and game engine. Moreover, a web browser actually has two components referred to as engines: the browser engine and JavaScript engine.
One seeming outlier is a search engine, such as google.com, because it is a stand-alone service provided to end users. However, for the search provider, the engine is part of a distributed computing system that can encompass many data centers throughout the world.