What characteristic(s) qualifies a beverage as a "cola"? Is it just a marketing term?

well..... I'd say dark color with a "cola taste". It probably should have Kola in it as well...... but, maybe not - I say that because the original formulation of Pepsi-Cola did not actually have any Kola nut extract in it. (or caffiene!)

according to wikipedia.org...

Cola - Wikipedia, the free encyclopedia

That's a good question. Up until the early '80's, the US FDA required anything sold as a cola to have caffeine. I don't know what they require now to call a drink a cola.