Engines are significantly higher level than what we call frameworks in web dev, with a lot of stuff built-in and coding being a minor requirement. Traditionally they were even genre-specific: fighting game in case of Mugen, shooter in case of Quake and Source, etc. Also, in the case of Mugen, they are mostly data-centric rather than code-centric like a web framework.

Another interesting distinction is that engines can often written in one language but offer extensibility in another, like being written in C++ and having programmers write Lua or C#. Notable exceptions exist, however. In frameworks that's quite rare I guess.

In addition to the high vs low level comments, I think another helpful connotation of the "engine" term is that you're often dealing with things that happen per frame, with many frames a second. The concept of an "engine" pumping out all these frames can be helpful.

This isn't to say that you don't write code which runs at framerate with a web framework, but often this is abstracted behind layout and user interaction concepts.

removed.