History: What is unique about a feedback delay?

What is unique about a feedback delay?

• A feedback delay isn't unique.
  They are common, but not all delays use feedback.
• A feedback delay plays the previous copies only.
  It's both!
• A feedback delay plays copies of the original sound only.
  It's both!
• A feedback delay plays the combination of the original sound source and the previous copies as well.
  Good job!