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

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.