清真 refers to Islam or what is permissible under Islamic law, especially food (halal). The term literally means 'pure and true' and is used as a modifier in words like 清真寺 (mosque) and 清真食品 (halal food). Though POS is marked as a noun, in practice it often appears attributively (like an adjective) before another noun. The historical sense 'purity' or 'kosher' is now very rare; modern usage is almost exclusively about Islam.