From AMS Glossary
Old snow that has become granular and compacted (dense) as the result of various surface metamorphoses, mainly melting and refreezing but also including sublimation.
The resulting particles are generally spherical and rather uniform. Firnification, the process of firn formation, is the first step in the transformation of snow into land ice (usually glacier ice). Some authorities restrict the use of firn to snow that has lasted through one summer, thereby distinguishing it from spring snow. Originally, the French term, "névé," was equivalent to the German term, "firn," but there is a growing tendency, especially among British glaciologists, to use "névés" for an area of firn, that is, generally for the accumulation area above or at the head of a glacier