Mechanical Extract Ventilation, also known as MEV, is a centralised ventilation system that provides constant and continuous ventilation, drawing excess moisture from multiple rooms throughout the property and ensuring good indoor air quality throughout.  

Because the system is constant, it provides much better protection against condensation and mould than the traditional extractor fans. 

A further level is Mechanical Ventilation and Heat Recovery (MVHR), much the same in principle to MEV, except for the additional process of extracting heat from the moisture pulled from bathrooms and kitchens, and then transferring the heat into the fresh filtered air it supplies to the rooms. This makes for a much more energy efficient house. 

Both systems provide a high-end solution to the ventilation needs of a property, but both are more appropriate in new build properties than converted properties, MVHR especially so that they can ensure that most air flow is passing through the heat exchanger.