深层 literally means 'deep layer'—it combines 深 (shēn, 'deep') and 层 (céng, 'layer' or 'level'). While it can refer to physical depth (e.g., deep layers of soil or ocean), it is most commonly used metaphorically to describe abstract, underlying aspects: deep-seated emotions, subconscious motivations, fundamental causes, or foundational principles in science, psychology, or medicine.

This term appears frequently in academic, medical, and technical contexts—for instance, '深层组织' (deep tissue) in physiotherapy or '深层原因' (underlying cause) in social analysis. It carries a formal, precise tone and rarely occurs in casual speech. Unlike the simple adjective 深, 深层 functions almost exclusively as a noun or attributive noun (e.g., 深层结构, 'deep structure'), often modified by classifiers like '一个' or preceded by adjectives like '最' (most).