Sky Replacement
Substituting the sky in a photograph with one from another image, with automatic masking and colour matching.
In depth
Modern implementations mask the horizon automatically and relight the foreground to match the new sky's colour and direction, which is what makes the result convincing. It raises a disclosure question rather than a technical one: acceptable in commercial and creative work, and a serious breach in documentary, journalism and competition.
Examples in practice
An estate agent's exterior shot has its grey overcast replaced with light cloud, with the foreground warmed to match.