A visible trail that is left by or indicates a projectile's flight path. The most common example is the glowing trail of speeding bullet, which in real life is done by firing tracer ammunition. (see Danbooru post #6371369)
This applies to any form of projectile (e.g. arrow on Danbooru post #6271456 or throwing knives on Danbooru post #4981382), and somehow can be rendered as a subset of speed lines.
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