What does invertebrate mean?

The term "invertebrate" refers to animals that lack a backbone or spinal column. This characteristic distinguishes them from vertebrates, which are animals with a spine, such as mammals, birds, reptiles, amphibians, and fish. Invertebrates make up a vast majority of animal species on Earth, including groups like insects, arachnids, mollusks, and crustaceans. Understanding this definition is crucial, as it helps categorize animals based on their anatomical features, which ultimately informs their biology, behavior, and ecology. The other options describe different concepts but do not accurately define what makes an invertebrate.