An option that can only be exercised when the price of the underlying asset reaches a set barrier level. This is a type of a knock-in barrier option. 
Investopedia Says:
The prices of these options tend to be lower than "vanilla options" as the ability to exercise the option is contingent on breaking the barrier.
Related Links:
An introduction to the world of options, covering everything from primary concepts to how options work and why you might use them. Options Basics Tutorial
