Answer:
A browser engine is not a stand-alone computer program but a critical piece of a larger program, such as a web browser, from which the term is derived. (The word "engine" is an analogy to the engine of a car.)
Besides "browser engine", two other terms are in common use regarding related concepts: "layout engine" and "rendering engine". In theory, layout and rendering (or "painting") could be handled by separate engines. In practice, however, they are tightly coupled and rarely considered separately.
In addition to layout and rendering, a browser engine enforces the security policy between documents, handles navigation through hyperlinks and data submitted through forms, and implements the Document Object Model (DOM) data structure exposed to page scripts.
