Obstacle avoidance is a very similar concept to the Separation rule. The only real difference is that it is used to avoid obstacles in the world rather than members of the flock.
Obstacle avoidance is different from a steering behaviour like Flee in that corrective steering will only take place if required.
During simulation, boids look along their current trajectory to see if there is likely to be a future collision with an obstacle. If a collision is predicted they will take evasive action to avoid that collision. However if no potential collision is detected, they will not change course.