Looks like I voted that question down not so long ago. There are a couple of problems. First, this part:
This should go without saying, but please base your answer on Scripture and prove your claims using Scripture. (By "Scripture" and "Bible" I mean the 66 books of the Protestant Bible.)
This looks like you're using the question to make a controversial statement about which books in the Bible are canonical. Including this is very unusual among all the "biblical basis" questions on this site. Do you have any reason to believe that the mention of a few passages from non-Protestant books would produce bad answers? You might say "I understand the book of [...] actually defines what prophesy is, but it is not part of the biblical canon that I accept, so I would like to know if this is discussed elsewhere". This would count as "research effort", and would explain why you choose to bring up the issue of biblical canon and why restricted your question to exclude over half of Christianity.
Look at this question (the title is sufficient):
Perhaps you too are left wondering how a person marginally familiar with Christianity could end up thinking that the passage was describing a miracle (a violation of the laws of nature). I have no idea why that's in there or why the question even exists in its present form. If my response to the question is a baffled what were they thinking?, then the question lacks research effort. Here I conclude that the author is (perhaps unintentionally) making the statement "I expect Christians to explain confusing things away by calling them miracles".
You're obviously familiar with Christianity, and in your question the part in parenthesis is not that bad, but I still have no idea why you included it. Get rid of it. As a bonus, more people will upvote due to interest, more people will answer.
The second problem is that you're asking for a definition from the Bible, and everyone seems to be ignoring you. The Bible probably does not have a very direct definition of what counts as prophesy. Actually, it looks like the Bible isn't really a source of definitions:
This is why one of the answers is relying on a non-religious dictionary, and the others are reasoning a definition out from scripture. So get rid of the word "definition" in the body of your question ("What does Scripture say prophesy is?"). But keep the title, because the question is now the top search result for 'bible definition prophesy'.
Alternatively, you could replace the bold part with "what are the different definitions of 'prophesy' as used in Scripture", since there are potentially different uses of the term.
(And get rid of the word "Thanks." It's nice to see, but I wouldn't want to see it on every question.)