Rehab or rehabilitation is having something restored. The most common use of this as we can hear it from the news is rehab due to the use of illegal drugs. The term rehab in this context is to restore or regain the person's health condition and have it in its normal state just before taking drugs. Or say the Old churches were devastated by the earthquake and it needs rehabilitation. It only means that the churches should be rebuilt and have it in its original state just before the earthquake strikes.