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 post #6371369)
This applies to any form of projectile (e.g. Arrow (Projectile) on post #6271456 or throwing knives on post #4981382), and somehow can be rendered as a subset of Speed Lines.