Overlay means to place one thing on top of another, often so that parts of both remain visible. When you overlay a map with a sheet of tracing paper to copy specific features, you're putting one layer over another. In art class, you might overlay watercolors, painting one transparent color on top of another to create new shades.

In technology, an overlay is something displayed on top of another image or screen. Video games often use an overlay to show your health bar, score, or inventory without blocking the main game view. When you watch a football game on TV, the score and clock appear as an overlay on the broadcast. Weather forecasters use overlays to show radar images on top of maps.

The word suggests that what's underneath stays visible or important, which distinguishes it from simply covering something completely. When you overlay information, you're adding new details while keeping the original context. An architect might overlay building plans on a photograph of a vacant lot to show clients what the finished structure will look like. A scientist might overlay graphs from different experiments to compare results.

As a noun, an overlay is the layer itself: “The graphic designer created an overlay to highlight important regions on the chart.”