Is "coupling" a noun or verb?

Great question! In short, yes and yes... but really, "coupling" is a gerund. 

In an earlier post, we determined that... as much as many like using the term "coupler"... "coupling" is the appropriate term for describing the physical product connecting/joining two shafts together... but doesn't "coupling" also sound like a verb? (Perhaps this is the root of why some people opt to using "coupler" to describe a "coupling"?)

So what exactly is a gerund? Per Wikipedia, "a gerund is a non-finite verb form that can function as a noun in Latin and English grammar. The English gerund ends in -ing (as in I enjoy playing basketball); the same verb form also serves as the English present participle (which has an adjectival or adverbial function), and as a pure verbal noun. Thus the -ing form in the English language can function as a noun, verb, adjective or sometimes adverb; in certain sentences the distinction can be arbitrary." There you have it... "coupling" is a verb that functions as a noun.

As couplings play such an instrumental role in the life and health of a mechanical power transmission system, I certainly can blame anyone from trying to give the product a noun only name (thus the "coupler" camp)... but I'm also not so sure being a gerund is such a bad thing. It's fun, unique and something I either never learned or quickly forgot during high school English!