The brain, or specifically the main cavity or ventricle within the skull; used mainly in older medical texts.
From Greek enkephalos, where en (in) + kephale (head) literally means 'in the head.' This ancient Greek term was the direct predecessor of all modern 'encephalo-' medical terms.
The word 'encephalon' is the original Greek term—when scientists wanted to describe brain-related things, they literally used the ancient word for 'the thing in your head,' which is beautifully logical.
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