Does it do one thing? It's probably a library.
Is it a collection of libraries that work together focusing on primitives with callbacks? It's probably a framework.
Does it load levels? Have its own schema for an entity? Provide a multiplayer server protocol, and build on primitives? It's probably an engine.
Libraries -> frameworks -> engines -> games.
A really, really well designed engine is just going to be a library of highly modular components with a nice editor or two thrown in.
As the old adage goes "Build libraries not frameworks"
Does it do one thing? It's probably a library.
Is it a collection of libraries that work together focusing on primitives with callbacks? It's probably a framework.
Does it load levels? Have its own schema for an entity? Provide a multiplayer server protocol, and build on primitives? It's probably an engine.
Libraries -> frameworks -> engines -> games.