An overlay can refer to different things depending on the context. Here are some definitions of overlay based on the search results:

• General Definition: Overlay means to lay or spread over or across something, or to cover, overspread, or surmount with something.
• Video Editing: In video editing, overlay refers to a technique where multiple video clips or images are layered over one another to create a single video. The overlay can be partially transparent to allow the viewer to see the original video underneath, or it can completely cover the original video. The size, position, and duration of the overlay can be adjusted according to the editors needs. This technique is a fundamental part of post-production and is used in virtually all types of video production, from movies and TV shows to online videos and commercials.
• GIS and Mapping: In GIS and mapping, overlay is a procedure that estimates the attributes of one or more features by superimposing them over other features, and figuring out the extent to which they overlap. Overlays are used to combine different layers of geographic data to create new layers that show the relationships between the original data sets. For example, overlaying a layer of population data with a layer of land use data can show which areas are most densely populated.

In summary, an overlay can refer to a technique used in video editing, a procedure used in GIS and mapping, or a general term for laying or spreading something over or across something else.