Jpmedia: Responsa Radio

I recently heard the idea that if the reason behind a rabbinic ruling (gezeirah) becomes irrelevant, because the surrounding factors are no longer applicable, the decree no longer applies. This sounds like it could undermine a lot of halakhah in modern times. What is the definition of this concept how and when can it be applied? Have a question you'd like answered on Responsa Radio? Email us at responsa@hadar.org.