The small permanent body of a comet, believed to have a diameter between one and a few tens of kilometers, and to be composed of water and volatile hydrocarbons.

1. That portion of the control program that must always be present in main storage.

2. The main storage area used in the nucleus (first definition) and other transient control program routines.

A small mass of differentiated protoplasm rich in nucleoproteins and surrounded by a membrane; found in most animal and plant cells, contains chromosomes, and functions in metabolism, growth, and reproduction.

A mass of nerve cells in the central nervous system.

A particle of any nature upon which, or a locus at which, molecules of water or ice accumulate as a result of a phase change to a more condensed state.

The central, positively charged, dense portion of an atom. Also known as atomic nucleus.

A central mass about which accretion takes place.