Core Electron
Core electrons are the electrons in an atom that are not valence electrons and therefore do not participate in bonding. An example: the carbon atom has a total of 6 electrons, 4 of them being valence electrons. So the remaining 2 electrons must be core electrons.
They are so tightly bound to the nucleus as to be negligibly perturbed by the environment of the atom when in the solid state. Therefore on the contrary of the valence electrons, the core electrons usually play a secondary role on chemical bonding and reactions and their main role is to screen the positive charge of the atomic nucleus. In transition metals, however, the distinction between core and valence electrons is more subtle and it could be very important to consider the electrons in the highest d-shells as valence rather than core electrons.
Read more about Core Electron: Electron Transition
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