A material that could turn from insulant to conductor on-the-flight, that means during its operation. Semicondutoros are like that but their nature is defined in the factory, during manufacturing. 

If you come to think about it, switches are this, but they are approximated by hard 100% condutive or 100% insulant. I'm proposing something that could go any value in between.

In fairness, I coudn't come up with an application. It just sounds very cool.