As a noun, it is the central or most important part of something, like the core of an apple or the Earth. As an adjective, it describes something basic and essential.
Possibly from Old French *cor* 'heart', from Latin *cor, cordis* 'heart'. The idea shifted from the physical heart to the central part of anything.
Core is your 'heart layer'—core values, core muscles, core ideas. In physics and geology, the core is literally the intensely hot center of planets or stars. The word reminds you that most systems, from bodies to beliefs, have a central part that holds everything together.
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