Shows how accurate a warrior is with ranged weapons such as guns and energy weapons. It also includes a measure of the warrior's technical competence with their weapons. The higher this score is the easier a model finds it to hit when it shoots at something. Some monsters have natural weapons that can be used at range (they may be able to spit venom, for example) and their BS is used to determine whether they hit or not.
Ballistic Skill
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 2