This is the name of a filter range created by filter maker H&Y filters. Instead of using a filter thread of a fixed size, it houses a spring-loaded mechanism that can adapt to a wider range of filter sizes, making it possible to use one filter with a different number of lenses. You still have to choose different Revoring sizes to handle small, medium and larger lens sizes.
The Revoring range includes regular round filters using this design, including combined variable ND and polarizing filters, black mist filters, and ’empty’ Revorings which can be used as multi-size step up rings. H&Y has also experimented with magnetic filter systems using regular square filters.