Firn is partially compacted névé, a type of snow that has been left over from past seasons and has been recrystallized into a substance denser than névé. Wikipedia

A type of old snow which has gone through multiple thaw and refreeze cycles and thus is made of numerous small icy grains, though it is not nearly as saturated with water as snow-cone slush is; can be hard or somewhat soft depending on recent and current weather conditions. Wiktionary

A type of old snow which has gone through multiple thaw and refreeze cycles. Wiktionary (translation)