Is there a standard dictionary for referencing English words?
www.oed.com is the online version of the full, official Oxford English Dictionary. Requires a subscription (institutional or personal) to access. This is the site you should use whenever possible.
http://www.oxforddictionaries.com is an ad-supported version with some features cut. Avoid if you have subscription access to the full site (as any university should).
http://oaadonline.oxfordlearnersdictionaries.com/ is, clearly, for learners of the English language. Use it as a guide for learning, sure - but not as a guide for writing papers.


The OED is the English dictionary to use. Other dictionaries are probably fine in all but the weirdest corner cases, but it helps to have some standardization.

Your journal probably specifies its own preferences in this regard. Every publisher's "guide for authors" that I have seen tells you to defer to the OED. Sometimes you'll see specific instructions to use either British or American spellings, and you may specifically be told how to spell words that don't have a British or American "standard" spelling, such as parametrize.