quantum tunneling

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A quantum mechanical effect in which particles have a finite probability of crossing an energy barrier, such as the energy needed to break a bond with another particle, even though the particle's energy is less than the energy barrier. The emission of particles from an atom's nucleus in radioactive decay is a case of quantum tunneling; the phenomenon is also exploited in the design of many electrical components.