Is a backpack not a carry-on?

Excuse me, but could you please clarify something for me? I've been wondering about the distinction between a backpack and a carry-on. Specifically, I'm wondering if a backpack isn't technically considered a type of carry-on luggage. After all, both are designed to be portable and easily transported, often used for travel. So, is there a specific reason why a backpack wouldn't be classified as a carry-on, or am I misunderstanding the terminology? I'd appreciate your insights on this matter.