uPVC is the material that the frame of the double glazed windows is made off. This material is based on plastic powder (unplasticized polyvinyl chloride, uPVC), which is heated up and injected into a mould in order to form its shape. After it has cooled through different methods, the uPVC is cut and prepared to be assembled in a double glazed window together with the other components.
uPVC is not the only material that double glazed window frames are made of. They can also be made of timber and aluminium. All three materials come with different advantages and disadvantages, but overall, uPVC is the most used.