A quantum mechanical phenomena resulting from photon delocalization [1] that allows light to cross propagation barriers such as an interface. Sir Isaac Newton was perhaps the first to experimentally observe and document optical tunneling [2]. More recently the principle has been used to surpass two theoretical limits of light, namely the "speed limit" and the "far-field diffraction limit."
References:
[1] Ole Keller, 'Near-field optics: The nightmare of the photon' The Journal of Chemical Physics (2000)112, 18, 7856.
[2] Newton measured optical tunneling in the 17th century by observing ring patterns that appeared when he placed a prism with a planar interface in contact with one with a spherical interface. Although quantum mechanics was not yet developed at that time, Newton found a way to model the phenomena using his corpuscular theory of light in the historical book entitled Opticks.