Open English Wordnet
Nouns
(n)core, nucleus, core groupa small group of indispensable persons or things“five periodicals make up the core of their publishing program”
(n)kernel, substance, core, center, centre, essence, gist, heart, heart and soul, inwardness, marrow, meat, nub, pith, sum, nitty-grittythe choicest or most essential or most vital part of some idea or experience“the gist of the prosecutor's argument”, “the heart and soul of the Republican Party”, “the nub of the story”
(n)core, magnetic core(computer science) a tiny ferrite toroid formerly used in a random access memory to store one bit of data; now superseded by semiconductor memories“each core has three wires passing through it, providing the means to select and detect the contents of each bit”