1. Altering the appearance of a photograph using paints or dyes which were often applied to photographic prints with an airbrush. Airbrushing was routinely used to remove unwanted blemishes in fashion magazine shoots, hence the connotation of presenting an unattainable idealized image. In Stalin's Russia, officials were removed from photographs altogether and thus airbrushed from history.
2. Loosely, any photographic alterations, including those done digitally. Compare photoshopping.