'Cloud layer' refers to a horizontal sheet or stratum of clouds at a specific altitude in the atmosphere. The character 云 (yún) means 'cloud', and 层 (céng) means 'layer', 'stratum', or 'level' — often used to describe stacked or stratified structures, whether physical (like rock layers) or abstract (like organizational levels). Together, 云层 emphasizes the layered, often uniform appearance of clouds when observed from the ground or aircraft.

This term is commonly used in weather reports, aviation, geography, and photography. It appears frequently in descriptions of sky conditions ('thick cloud layer', 'broken cloud layer') and atmospheric phenomena ('the cloud layer lifted at dawn'). Unlike more poetic terms like 云朵 (clouds as fluffy objects), 云层 carries a scientific or observational nuance, highlighting structure and vertical positioning rather than shape or beauty.