A
tracer round is a
projectile (usually for an automatic weapon) which has been treated with a substance designed to burn and leave a glowing, visible trail. It is used both for
aiming purposes as well as for psychological effect; the
AC-130H/U Gunship and
its ilk use large proportions of tracer in their ground-attack weapons in order to impress upon the intended targets just how much trouble they're in.
Tracers are usually treated with a strontium salt, which will burn (I believe) with a bright orange-red trail. Salts of other compounds can be used to provide tracer paths of different hues. Note that tracer rounds at best cause a requirement for more frequent cleaning of the weapon and at worst the premature erosion of the lands in the barrel and of the breech.