DissolveIn chemistry, to dissolve a substance is to cause that substance to pass into a solution. For example: common salt (sodium chloride) can dissolve in water.
In film, a dissolve is a gradual transition from one image to another. In film this effect is created by controlled double exposure from frame to frame; transiting from the end of one clip to the beginning of another.
In video the effect is created by interpolating voltages of the video signal.