贝壳 (bèi ké) literally means 'shell of a shellfish' — 贝 (bèi) originally referred to cowrie shells, which were used as ancient currency and symbols of wealth in China, and later came to mean 'shellfish' or 'mollusk' broadly; 壳 (ké) means 'hard outer covering' or 'shell' in general. Together, 贝壳 specifically refers to the hard, often colorful, calcified exoskeletons left behind by marine mollusks like clams, oysters, and snails after they die.

This word is commonly used in everyday contexts — from beachcombing and crafts to biology class and museum exhibits. It carries neutral, descriptive connotations and appears frequently in children’s books, travel writing, and environmental education. Unlike 壳 alone (which can refer to eggshells, nutshells, or even metaphorical 'shells'), 贝壳 always implies a marine origin and natural beauty, often evoking seaside imagery and collectible objects.