“Fluid polyamorphism” is the existence of two alternative amorphous structures in a single-component fluid. It is found in very different materials, such as silicon, phosphorus, cerium, and hydrogen, usually at extreme conditions. In particular, this phenomenon is hypothesized in metastable supercooled water, inaccessible for direct bulk-water experiments because it is predicted to be below the empirical limit of homogeneous ice nucleation. I present a generic phenomenological approach to describe polyamorphism in a single-component fluid, applicable regardless of the microscopic origin of the phenomenon. To specify this approach, I consider a fluid with “chemical reaction” equilibrium between two competing interconvertible states or structures. This approach for the physics of liquid-liquid separation in a single-component fluid is based on a discrete nature of two distinct structures. The approach qualitatively describes the global phase diagram of a fluid, with both vapor-liquid and liquid-liquid equilibria, as well as peculiar properties of polyamorphic fluids.