Pyknosis is a stage in programmed cell death in which the chromatin in the nucleus of a cell polymerizes and condenses into a solid structureless mass.
In the stages of cell death, this stage is preceded by karyolysis, the dissolution of the cell nucleus, and is eventually followed by karyorrhexis, or fragmentation of the nucleus.
Pyknosis is also observed as a natural part of the maturation process of erythrocytes (a type of red blood cell) and neutrophil (a type of white blood cell).
The word pyknosis comes from the Greek combining forms pykn-, meaning "thick, solid, dense", and -osis, meaning a medical condition. Since the Greek letter [k] was always rendered into English words as [c], the fact that the word "pyknosis" has a "k" in it tells us that this word was borrowed into English in relatively recent times from German, where Greek roots retained the "k".
This makes sense because the Germans were the creators of most modern medical terminology back when modern medicine was getting started in the 19th century.