Pre-rendered 2D graphic or animation layered multiple times without recalculation — foundation for particles, light flares, precipitation, debris.
You have particle simulations to render — thousands of raindrops, sparks, snowflakes. Recalculating each frame costs processing time you don't have. This is where sprites come in: a pre-rendered 2D graphic or short animation that you place thousands of times in the frame without recalculating it. A single image of a water droplet, a light ray, a smoke particle — that's your sprite. You place it in 3D space, scale it, rotate it, change its transparency, and the engine takes care of the rest.
In practice, it works like this: You generate your sprite textures beforehand — either photographically (real raindrops, smoke captures) or via CG (clean renderings of individual elements). You often pack multiple variations into a sprite sheet so that your particles don't all look identical. A raindrop sheet might contain ten different shapes. The engine randomly or sequentially selects from these during rendering, creating variation without real geometric complexity. For an explosion with a hundred thousand particles, this is the difference between 10 minutes and 2 hours of render time.
The catch: Sprites only function optimally from certain camera perspectives — they are always oriented towards the viewer (billboarded), unless you consciously need them as flat elements. At extreme camera angles or when sprites overlap, it can look unnatural. Therefore, sprites are often used for smaller, fast effects — light flares, glows, wisps of smoke — where the eye doesn't linger. For large, spatial effects (dense smoke volumes, water splashes), you mix multiple sprite layers or combine them with volumetric techniques.
In compositing or in a VFX engine (Houdini, Nuke), you work with sprite sequences — often exported as EXR sequences with an alpha channel. This gives you maximum control over timing, blending, and color correction. A well-built sprite system saves you not only on the render farm but also in the artistic feedback loop: you can quickly adjust parameters without re-rendering everything. That's the real advantage.