Definition of Literary

Sorry for the gap in posting!  School's start has been busy for me, but I did have a very interesting question posed to me in English class, which I wanted to share.  Imagine you've been assigned by Merriam-Webster to write a few definitions.  Your first assignment?  Define "literature."
At first, it seems easy.  Written word--but what about Us Weekly magazine, Twilight, even stop signs?  Did those count? So it needs amending.  But then you get too personal and too opinionated.   I thought about "written word that is meaningful to someone," and for a time was confident that I had found the answer.  Then my teacher asked if his email to his mom--"Mom, you're the best!"--counted as literature.  The class agreed that it didn't.  But it was still meaningful to his mother, he said.  Then I had to start over.
Who would have thought that such a simple word, one that I use, or reference, in almost every blog post, could have proved so inscrutable?  The problem lay in the fact that everyone has their own opinion of what counts as literature.  Some people stand by the classics, others argue that fandom-inspiring novels such as Twilight and the Harry Potter series can count as well, while some exhausted students just said written word and left the definition in its broadest form.  What do you think?   Is there a limit to what we can and cannot call literature?
For the record, Merriam-Webster's website has several definitions for the word, but I think the most fitting to this post is: "writings in prose or verse, especially writings having excellence of form or expression and expressing ideas of permanent or universal interest."  A clever definition in that it still leaves room for opinion.