For the purposes of Stack Exchange's Literature section, it is clear that any published work about which someone has a question, and seeks an answer here, is literature.
More broadly, “literature” is one of those words, like “love” and “art” which defy stable, accepted definitions. These are words which different people, different peoples, different eras define differently.
Take the Gospels and the Koran, for example. Many scholars of comparative religion might refer to both of them as literature. Many of those who regard them as sacred texts might bristle at lumping them together with “The Sun Also Rises” and “The Castle of Otronto”.
Personally, I bristle at including as “literature” many of the works discussed on this literature website – at the inclusion, for example, (looking at today’s questions) of Dr. Seuss and “The Phantom Tollbooth”. On the other hand, acquaintances of mine similarly bristle when I claim that “The Lord of the Rings” is literature.
Something else to ponder: I see that in 1828 Daniel Webster (or one of his minions) defined “Literature” not as a body of writing, but as a personal attribute.
LIT'ERATURE, noun [Latin literatura.] Learning; acquaintance with letters or books. literature comprehends a knowledge of the ancient languages, denominated classical, history, grammar, rhetoric, logic, geography, etc. as well as of the sciences. A knowledge of the world and good breeding give luster to literature.