It is a layer of ionised gas occurring at 90-150km in altitude, one of several layers in the ionosphere. It reflects medium-frequency radio waves, and because of this reflection radio waves can be propagated beyond the horizon.
Propagation is affected by time of day, season, and sunspot activity.
Nikola Tesla originally discovered this area. He made mathematical calculations based off his experiments in his research of electromagnetic propagation and resonance and deduced that the resonant frequency of this area was approximately eight hertz.
Its existence was proposed in 1902 independently and almost simultaneously by the American electrical engineer Arthur Edwin Kennelly (1861-1949) and the British physicist Oliver Heaviside (1850-1925).
The "Heaviside Layer" is used as a symbol for heaven (in the afterlife sense) in Andrew Lloyd Webber's musical Cats. This reference is based on a quote found in a letter written by T. S. Eliot, whose book Old Possum's Book of Practical Cats forms the basis of the musical.