Pyknosis, or karyopyknosis, is the condensation of chromatin in the nucleus of a cell undergoing programmed cell death (see Naoufal Zamzami and Guido Kroemer: "Apoptosis: Condensed matter in cell death", Nature Vol. 401 p. 127, 09 Sep. 1999 [1]).

It is followed by Karyorrhexis, or fragmentation of the nucleus.

See also:

• Apoptosis, especially the section on the morphology of apoptotic cells.
• Karyorrhexis