Any restrictions on engines?

[StevenB]

I love how the trolling thread has been derailed into something useful.

Let's make a list:
Motors
Electric motor
Rocket motor
Molecular motor
Motorcycle
Motor neuron

Engines
Gasoline engine
Jet engine
Steam engine
Stirling engine
Search engine

I remember reading a model rocketry book which was adamant that the propulsion was a "rocket motor", and not a "rocket engine". But I see now that Estes calls them "engines".
Wikipedia treats "motor" and "engine" as effectively synonymous, at least until someone here goes and edits the article.