What makes a line?

What is the most important criterion for a text's having a claim to being a poem? What if it's not a text? what if it's oral poetry, like Homer? What authorizes us to say that there are five feet in a pentameter line, or six in a hexameter, when Milton and Homer recite their verses orally, or Shakespearean actors utter blank verse soliloquies on stage? Are lines (unrhymed lines, anyhow) just artifacts of printing? Hint: no. Are they ever artifacts of printing? Hint: yes.