opaque
adj.adj. not letting light pass through, so you cannot see through it. It also describes writing or ideas that are very difficult to understand.
adj. impenetrable to light; lacking transparency or translucency. In abstract contexts, it refers to language or processes that are intentionally or naturally obscure and difficult to interpret.
The bathroom window is made of opaque glass for privacy.
The company's hiring process is notoriously opaque, leaving many applicants confused about why they were rejected.
While the physical properties of the mineral make it entirely opaque, its chemical composition remains a subject of intense debate among geologists who cannot peer inside the crystal structure.
From Middle English opake, from Latin opacus (“shaded, shady, dark”) (of unknown origin), later reinforced from Middle French opaque. Doublet of ubac.
Can be used both literally for physical objects and figuratively for abstract concepts like logic or systems.