To place something on top of another thing; also a layer of material or digital image placed over something else.
From 'over' + 'lay' (to place). Used since Middle English. In modern computing, 'overlay' describes graphics placed on top of video or maps.
Google Maps pioneered the 'overlay' concept by letting users toggle different map layers (terrain, traffic, satellite) on and off—this simple idea of stacking transparent layers revolutionized how we visualize complex information.
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