Crystal - A solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern such that regularly spaced atomic planes are found in three dimensions.
Firn - Compacted snow that has survived at least one melt season in a temperate glacier, or, in a Polar Region, compacted snow that underlies fresh snow, if any, and has undergone some metamorphism.
Firn-ice transition - The change from a packed granular material containing interconnected pores to a polycrystalline solid containing individual bubbles.
Grain - A small single piece of a substance. In firn, the term is used to refer to a single connected piece of ice, regardless of its atomic arrangement; hence, it could include one or more actual crystals. In polycrystalline ice, the terms crystal and grain are used interchangeably (Snow Grains).
Pore close-off depth - The depth at which only individual bubbles are found, rather than interconnected pores.
Snowpack - An area