without any mark, sign, or label; not identified or distinguished in any way.
From Old English 'un-' (not) plus 'marked' from Germanic 'mark' meaning 'sign' or 'border.' The word simply means lacking any identifying characteristic.
Police use unmarked cars based on 1960s psychology research showing that visible authority symbols change human behavior; when people don't know they're being observed, data collection is 'cleaner' and more honest.
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