a part of the cell containing DNA and RNA and responsible for growth and reproduction
any histologically identifiable mass of neural cell bodies in the brain or spinal cord
the positively charged dense center of an atom
(astronomy) the center of the head of a comet; consists of small solid particles of ice and frozen gas that vaporizes on approaching the sun to form the coma and tail
the branch of quantum physics that accounts for matter at the atomic level; an extension of statistical mechanics based on quantum theory (especially the Pauli exclusion principle)
the time required for something to fall to half its initial value (in particular, the time for half the atoms in a radioactive substance to disintegrate)