a dissolve (sometimes called a lap dissolve) is a gradual transition from one image to another. Most films dissolves are use to show a gradual pass of time in a montage.
There is an old saying between editors that if you cannot solve a cut you use a dissolve which means that you are unable to find a creative way to transition from one scene to another.
Dissolves needs a reason and a meaning to be use, a good example is the award winning editor Thelma Schoonmaker, longtime collaborator for director Martin Scorsese. Their work expand from Raging Bull to The Irishman, but one of the most beautiful piece of collaboration between Schoonmaker and Scorsese is the film The Age of Innocence
One of my favorite scenes of this movie is the transition ate the beginning of an event is held by Van der Luydens brings the characters of Archer and Countess Ellen Olenska.
Schoonmaker use the dissolve to build a crescendo in the preparations for the gala, every dissolve add a new element to the room where the gala takes place. Is a very simple transition, but is so well thought.
