a small group of indispensable persons or things

the positively charged dense center of an atom

any histologically identifiable mass of neural cell bodies in the brain or spinal cord

the central structure of the lens that is surrounded by the cortex

the central core of a comet, which is composed of ice, dust, and rocky materials

the central and typically sonorous part of a syllable, usually occupied by a vowel or a vowel-like sound