A phase splitter is a device that separates a signal into multiple phases (or polarities).
The term is most often applied to amplifiers that produce two "balanced" voltage outputs: of equal amplitude but opposite polarity (i.e. 180 degrees phase difference), but sometimes is used to refer to the generation of quadrature signals (i.e. differing by 90 degrees). The term is not used for logic circuits producing complementary outputs, nor applied to differential amplifiers that have balanced inputs and outputs.