I usually copy the google definition for common words, or link it if I'm using a specific dictionary.
But do all word definition answers need to have a dictionary reference link?
If you have copy/pasted a definition from somewhere, or simply copied it from a printed book, you must reference where you got the definition as well as providing a link back to the source where applicable.
Google definitions all come from a dictionary. Not always the same dictionary. That Google do not properly reference their sources is simply bad practice on their part. Luckily it is usually the first hit in the same set of search results.
For more information see What to do about missing source attributions?
If you copy something you must reference it.
Here's a moderator's comment to an answer deleted earlier this month:
This entire answer seems to be a verbatim quote from Cambridge Dictionaries. Mods are instructed to delete on sight without further warning any content that is not properly attributed. Moreover, if a question is sufficiently answered by a dictionary definition, it should not be answered in the first place, as it is off-topic as general reference. — RegDwigнt ♦ 13 secs ago
And in chat:
Do not edit. Delete on sight. Network-wide policy. If we get hit with a DMCA takedown notice, you are showing prior knowledge.
A link back is not mandatory, but it's recommended and helpful for online sources, especially as it allows others to follow up if they want more detail or to verify the source (for example, Google Books often reports incorrect metadata).