In my answer to this question, another user was adamant that Google is not a citable source and that that is a policy of ELU.
The link given as evidence for that was Is TFD a RTFM online source? - a footnote to an answer to a different question so I am not satisfied with that as an authoritative answer to this question.
Note that this and many other similar questions/answers were not just LMGTFY. The answer was the word (I didn't Google it to get it). The Google links are just convenient definitions of the word. This question is not about LMGTFY questions and answers. It is only about whether Google Dictionary is a valid reference.
I have searched on Help and Meta and cannot find anything that explicitly says it is or isn't.
There is the list of recommended dictionaries but it is only a list of recommendations not an approved list where anything else is forbidden. Google isn't on that list but neither is Collins for example.
What's up with all these dictionary reference edits? comes closest but there is a disagreement of opinions there.
There is speculation about where Google gets their definitions: some suggest they get them from various other online dictionaries (from leeching the definition from the first dictionary link). This doesn't seem to be supported by the wikipedia article on Google Dictionary
Google Dictionary was an online dictionary service of Google, originating in its Google Translate service.
The Google Dictionary website was terminated on August 5, 2011 after part of its functionality was integrated into Google Search using the define: operator.
...the content now came from another Oxford dictionary, the Oxford American College Dictionary. As of 5 August 2011, Google Dictionary was discontinued as a separate service, although similar results have been incorporated into the Google search sidebar.
So it seems that Google have their own dictionary, copied from one or more other dictionaries perhaps but, if so, they undoubtedly have the intellectual rights to do so. They provide this as part of a search (much like searching for "the time now" shows the current time) rather than as a separate URL as they did before. There is of course no plagiarism on their part. They cite no references and don't need to (just like other dictionaries).
Personally, I see no reason that Google Dictionary (now incorporated into Google Search) shouldn't be cited as a reference. So long as the definition it gives is copied into the answer (as all such references should) then any volatility of Google's dictionary is hardly an issue (the definition at the time of writing was shown). The source of the definition was Google and was cited. I see no problem with that.
There is a potential problem with Google search links in that many users copy a lot of unnecessary junk in the URL but this question only relates to properly sanitised Google search links: e.g. https://www.google.com/search?q=stack+exchange
So the question is in two parts:
1) Is there a policy on ELU that Google Dictionary is not acceptable for providing the definitions of words? (If so, please provide the link to where that is explicitly stated. Someone's opinion buried within a question on another subject on meta will not do.)
2) If not, should there be? (And why?)
I expect everyone who has an opinion has voted and the consensus is quite clear: Google dictionary is not to be referenced.
The down votes to the question are curious. Does that mean people feel it shouldn't have been asked or just that it was asked badly? I am glad it has been and I now know where ELU community stands on the question. I don't think it was clear before. People can say Google is not a citable source with a lot more confidence and authority now.