Encoder terminology x1 x2 x4
x1, x2, x4 encoding is commonly used, but most reasonable variations would be understood. English is kind of a free-for-all. 

In the case of higher end encoders that have analog sin/cos outputs, it's called interpolation. In the case of the (often German) manufacturers of such encoders, I do note that the newest generation cannot even agree on an interface standard so nomenclature is a pretty minor thing.