Photo manipulation is the altering of photographs to add, remove or alter something in the scene with an intention to deceive the viewer. A shortened for photo manip or just manip is used in the trade. In recent years it is most often done digitally by using software on a computer. Photoshop is a well known program that is used commercially to edit photographs.
Alterations like removing power lines or removing skin defects from models are not considered photo manips by the trade. This is airbrushing, a relic term from the days when photos were manipulated with a pneumatic air-powered spray brush that emitted a concentrated mist of paint.
An example of a photo manip would be placing one persons head on another persons body.