• 秘密ひみつ — secret
• 密室みっしつ — sealed room
• 密接みっせつ — close; intimate
• 親密しんみつ — intimacy; closeness
• 過密かみつ — overcrowding

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密 combines 宀 (roof) with 必 (essential), forming the image of a concealed or inner place — conveying secrecy, density, or something packed tightly.

It became widely known in 2020 with the phrase “3密” (san-mitsu: closed spaces, crowded places, close-contact settings) during the COVID-19 pandemic.