Flywheels are springs? ;)

(I think that coming closer to a definition of spring would be "stores mechanical energy through compression and releases it through expansion").

I think that you could somewhat legitimately differentiate between an object that stores mechanical energy and releases it due to its atomic structure (ie, every object), and one which stores and releases it at an improved ratio due to macroscopic structure, such as a, well, traditional spring.

If we're nitpicking, you can use a spring backwards too (but don't expand it too much, or it won't return)