Spiroid epsilon wave
Spiroid epsilon waves are a phenomenon encountered by the colonists of the planet Terra 10, where they were part of the natural environment.

When spiroid epsilon waves collided with matter, their spiroid particles produce a coiling effect that causes the distance between the atoms of the matter to be gradually reduced. When the Enterprise encountered the waves, the effect on their dilithium crystals rendered the ship virtually powerless. 

The waves eventually began to affect the crew as well, shrinking the space between the atoms of their DNA and reducing them to 1/16th of an inch in height, while the weight and mass of the crew remained unchanged.

It was soon discovered that the ancestors of the inhabitants of Terra 10 had also been affected by the wave after their colonization of the planet. Prolonged exposure to the waves rendered its effects permanent and the DNA of their descendants had become permanently altered so that miniaturization was now a genetic characteristic of the population. 

James T. Kirk initially referred to the waves as "x-waves". ()

See also
Epsilon radiation
Helical trauma
Radiation