Definition of core noun from the Oxford Advanced American Dictionary

        

    core

     noun
    noun
    NAmE//kɔr//
     
     
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  1. 1the hard central part of a fruit such as an apple, that contains the seeds
  2. 2the central part of an object the earth's core the core of a nuclear reactor The standards of housing and education are often lower in the older urban cores (= the centers of towns or cities).
  3. 3the most important or central part of something the core of the argument Concern for the environment is at the core of our policies.
  4. 4a small group of people who take part in a particular activity He gathered a small core of advisers around him.
  5. Idioms
    to the core
     
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    so that the whole of a thing or a person is affected She was shaken to the core by the news. He's a politician to the core (= in all his attitudes and actions).
See the Oxford Advanced Learner's Dictionary entry: core