牖

• Concrete: A window or wall opening used for light and ventilation.
• Metaphorical: An opening toward insight, instruction, or spiritual awakening; specifically the medium or access through which enlightenment is achieved.

Primarily a noun. May occasionally function as a verb implying "providing an opening" or "allowing access," though this is context-dependent and rare.

Compared to 窗 (chuāng), 牖 appears in earlier texts and more formal registers, typically referring to simpler architectural openings. 窗 became dominant in later periods, often implying a more developed structure with frames or shutters.

The reading is stable across Fanqie (與久) and Tang-period records (iǒu).