To be slightly more specific, it is not that it is poorly defined, it is that it doesn't exist. The boundary between liquid and gas is defined by a boundary at

To be slightly more specific, it is not that it is poorly defined, it is that it doesn't exist. The boundary between liquid and gas is defined by a boundary at which it requires additional energy and an accompanying abrupt change in density to change between the states. In a supercritical state, there is no additional energy or change in density. In other words, you can smoothly transition from "liquid" to "gas" without needing to boil. The two states are a single phase.