The line of departure is an extrapolated straight line extended in front of the gun in line with the bore axis at the moment the projectile leaves the muzzle.

In other words, if the bullet traveled in a perfectly straight line (which it does not) the line of departure is the path it would take. It could also be defined as the tangent to the trajectory at the muzzle.

related:

• trajectory

Footnotes / Sources:

“Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.