1. The crossing of a potential barrier by a particle that does not have sufficient kinetic energy to surmount it. The effect is explained by wave mechanics. Each particle has an associated wave function that describes the probability of finding the particle at a particular point in space. As a particle approaches the potential barrier the wave function is considered to extend inside the region of the potential barrier. Provided that the barrier is not infinitely thick a small but finite probability exists that the particle will appear on the other side of the barrier. As the thickness of the barrier decreases the probability of tunnelling through it increases. The tunnel effect is the basis of field emission and the tunnel diode. See also quantum mechanics.
2. The encapsulation of a packet from one protocol within the data payload of a packet from another protocol. This can allow nodes to use one protocol stack to communicate, even though the intermediate nodes only recognize another set of protocols.