A cross dissolve is a post-production video editing technique in which you gently increase the opacity of one scene over the previous one. In contrast to the jarring transition of a plain jump cut, one scene fades into the next, and the two images briefly overlap.
Cross dissolves, sometimes called cross-fades, were at their most popular during the early days of cinematography and can be considered one of cinema’s earliest special effects. In older movies, they were often used to show the passage of time, create a montage-like effect, or provide a gentle transition from one scene to the next.