From AMS Glossary
A phenomenon in which sound or radio energy may be detected only at various distance intervals from the energy source, as the result of the presence of an energy reflecting or refracting layer in the atmosphere.
For long radio waves, the ionosphere acts as the reflecting layer. For shorter wavelengths, the effect may be produced by strong superstandard propagation in elevated layers of the troposphere. Skip effects make it possible on occasion to detect targets at distances far greater than the normal radio horizon while closer targets remain undetected.