Definition of core noun from the Oxford Advanced Learner's Dictionary

        

    core

     noun
    noun
    BrE BrE//kɔː(r)//
     
    ; NAmE NAmE//kɔːr//
     
     
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  1. 1  the hard central part of a fruit such as an apple, that contains the seeds
  2. 2  the 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 centres of towns or cities).
  3. 3  the 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. see also hard core
    Word Origin Middle English: of unknown origin.Extra examples A hard core of supporters, mostly teenage girls, gathered at the airport to see the star arrive. A new spirit welled up from the very core of the nation. At the core of our convictions is belief in individual liberty. Each fibre has a hollow core trapping still air and aiding warmth. Each fibre/​fiber has a hollow core. He’s a politician to the core. Students study five core subjects. There was a hollow core of sadness inside me. These ideas formed the core of his philosophy. This is seen as the central core of the government’s policy. We need to stop this expansion and concentrate on the core business. We want to get to the core of the problem. a common core of shared understanding about law and government hard-core party members heat from the earth’s core nuclear reactor cores the core area of management studies the emotional core of her music Maths, English and science are the core subjects. This report goes to the core of the argument. What are the core activities of the job?Idioms 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 American Dictionary entry: core