What is an Amphibian?

What makes an amphibian an amphibian? Do amphibians lay eggs? Most do, but birds and fish lay eggs, too, and they aren't amphibians. Are amphibians animals whose young look totally different from the adults they will become? Yes, but what about butterflies and ladybugs? Their young also change shape as they grow up, but they're not amphibians. So what exactly is an amphibian? Answering that question isn't as easy as it might seem, but What is an Amphibian? makes solving the puzzle fascinating and fun.