firn

This term is a specialized glaciological descriptor used to identify a specific state of snow metamorphosis. It describes a material that is denser than fresh powder but not yet as crystalline or transparent as true glacial ice. Because it represents a transitional phase, it is typically used in technical contexts such as mountaineering, geology, or climatology.

As a mass noun referring to a specific type of substance, it is treated as uncountable. It describes the collective material of a snowfield rather than individual units, meaning it does not typically take a plural form in professional scientific discourse.