What characteristic defines miscible liquids?

Miscible liquids are defined by their ability to mix in any proportion without separating into distinct layers. This means that when two miscible liquids are combined, they thoroughly blend to form a homogeneous solution. An example of this would be water and ethanol; they mix entirely, demonstrating that they do not create separate phases.

The other characteristics listed do not apply to miscible liquids. If liquids are completely insoluble, they would not mix at all. Different densities might cause two liquids to separate, but it does not determine their miscibility. Similarly, forming layers in a container is indicative of immiscible liquids, which do not mix and instead remain distinct from one another. Thus, the defining characteristic of miscible liquids is their seamless mixing without separation.