底层 (dǐ céng) literally combines 底 (dǐ), meaning 'bottom' or 'base', and 层 (céng), meaning 'layer' or 'level'. Together, it refers to the lowest level of a building — most commonly the ground floor in Chinese-speaking regions (especially mainland China), though context may make it mean 'basement' (e.g., in shopping malls or older buildings with underground levels). Unlike English, where 'ground floor' and 'basement' are distinct, 底层 is context-sensitive: in residential high-rises, it usually means the first floor above ground; in commercial complexes with subterranean parking, it may refer to the lowest accessible level, even if below street level.

This term is neutral in register and widely used in real estate, architecture, navigation, and everyday directions. It’s also occasionally used metaphorically (e.g., 底层民众 — 'people at the bottom rung of society'), but that usage is more formal or literary. Learners should pay attention to context and building type when interpreting whether it means 'ground floor' or 'basement'.