The outer layer or membrane that forms the boundary of a cell.
From Greek 'kytos' (cell) and 'derma' (skin). Used since the 1800s in cell biology to describe what we now more commonly call the cell membrane or plasma membrane.
This is an older scientific term that fell out of fashion once we realized the cell membrane wasn't just a simple skin but a complex structure with proteins and careful regulation. Language in science often changes as we understand things better.
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