There seems to be some discussion about this on meta but no definitive answers.
This question says that "A verbatim copy-paste from an online dictionary is unnecessary, only the link has to be provided." I thought that made sense but am being told that's wrong.
This answer suggests that "a whole main entry from a reference work" is too much.
When one person contacted OED, they clearly said "It is fine to link to the OED, but not quote it at all." Tim claims OED is wrong but cites no more authoritative source. I agree with Trlkly's statement that some clear policy is needed.
Frank says "I now no longer add definitions copied from any copyrighted source, if I need to I will provide a link to the source (without including any material)."
Note that this meta question isn't about off-topic questions like "what's the meaning of this word that I could have easily looked up in a dictionary" but for questions like single-word-requests that are a bit harder to look up answers to via common reference sources. It seems unnecessary to paste in a definition especially when one can read the question or click the links to see what the word means.
I understand that link rot is generally an issue, though it seems like it should be less of an issue with dictionaries, and the link also always contains enough information that if a dictionary did change its format, one still has the word one's trying to look up and could just paste that into the search box. Batch editing for dictionaries that change URL formats could be possible too.
Dan Bron points to a post which says that answers should contain an answer and not just a pointer. I think that's not a fair criticism for the answer that comments on, which does contain the answer (the words requested). Had I just written something like "Try this, this, or this" that would probably fall short of actually providing an answer, especially if the word wasn't obvious in the URLs. A reader can look up the provided terms in almost any English dictionary or follow the links for one particular convenient dictionary.
In conclusion, I think that links to dictionary entries should generally be acceptable and (for copyright reasons) maybe even preferred over versions with pasted definitions as long as the meaning is clear in contexts like this when the word definition is a very close fit to what's being requested in the question. However, because some other far more experienced folks disagree, I'm bringing this up for focused discussion here.