Short answers, pulled from the story.
The word epic comes from the Latin epicus, itself derived from the Ancient Greek adjective epikos, which traces back to epos, meaning word, story, or poem. In Ancient Greece the term functioned as a noun referring to a long oral poem about a hero's adventures; over time it broadened into an adjective describing anything ambitious in scope or scale.
The Epic of Gilgamesh is the first recorded epic poem, originating in Mesopotamia and predating Ancient Greece and the Hebrew Bible. It is considered the foundation of the Western epic tradition and its themes, including a universal flood, a tree of life, and a hero challenged by a divine opponent, can be traced into the Bible, Greek, Buddhist, Hindu, Egyptian, Roman, and Zoroastrian traditions.
An epic is defined by its length relative to other works in its medium, a serious and formally elevated style, an epic hero who represents their community, a conflict whose stakes extend to the entire world of the narrative, and a mythos drawn from existing cultural narratives and traditions. These traits derive primarily from ancient oral poetry, particularly Homer's Iliad and Odyssey.
Folk epics are the earliest form, performed and transmitted orally, often in royal courts, with authors who are unknown and presumed to have been common men. Literary epics preserve these stories in written form, are more polished and compact in structure, and have identifiable authors. The Iliad and the Odyssey mark the transition between the two forms.
Epic fantasy, also called high fantasy, must meet three criteria: it must be a trilogy or longer, its time-span must cover years or more, and it must contain a large back-story or universe setting. J. R. R. Tolkien's The Lord of the Rings is a canonical example, while One Thousand and One Nights and the Indian epics Ramayana and Mahabharata show the genre extends beyond Western tradition.
The Arthurian legend was originally rooted in pre-Christian Celtic folklore and may have been based on a real British warrior of the 5th-6th century who resisted Saxon invasions. As the British Church grew in influence, authors reshaped the stories with Christian undertones. Robert de Boron translated the legend into French in 1155, adding the sword-in-the-stone and giving Arthur twelve knights to mirror Jesus's twelve disciples.