Final answer:
The major characteristic of arthropods is a chitinous exoskeleton which provides protection and is periodically molted to allow for growth. Other features include segmented bodies, jointed appendages, and adaptations for respiration.
Step-by-step explanation:
A major characteristic of arthropods is a chitinous exoskeleton. The exoskeleton of arthropods is composed of chitin, which is a waterproof, tough polysaccharide that provides protection and prevents dessication. Arthropods exhibit a variety of defining features, including a segmented body, jointed appendages for locomotion, sensory reception, feeding, and defense, a sophisticated nervous system, and respiratory structures like gills, tracheae, or book lungs. They undergo molting, which is the shedding of their exoskeleton to allow for growth. This is known as ecdysis and is a feature shared across members of the Ecdysozoa superphylum, leading to the incredible diversity seen within the phylum Arthropoda.