素 (sù) as an adjective means 'plain, simple, unadorned' — it describes clothing, decoration, or lifestyle that is basic and without extra ornament. 朴素 (pǔ sù) is a near-synonym but emphasizes a modest, frugal quality (e.g., 朴素的生活 'simple life'), while 素 is more neutral and often used for color or style (e.g., 素色 'plain color'). As a noun, 素 means 'vegetarian food' and is interchangeable with 素食 (sù shí) in everyday speech.