Something that can significantly hinder or completely block the ability to implement the Injection.

Difference between an Obstacle and a Negative Branch Reservation (NBR):

Obstacle BLOCKS OUR WAY to implement an Injection, and

NBR may appear as a SIDE RESULT of implementing the Injection.

Term_ENG_Obstacels vs NBRs

Source: Oded Cohen and Jelena Fedurko, Theory of Constraints Fundamentals, 2012