Very dense ice with few bubbles and a watery hue

Pure ice in the form of large crystals. It is blue owing to the scattering of light by the ice molecules; the purer the ice the deeper the blue.

Blue ice in the context of aviation refers to ice formed by leaks in commercial aircraft lavatory waste tanks, a concoction of human waste and liquid disinfectant that freezes at high altitude. The name comes from the blue color of the disinfectant, and as a sardonic reference to the Blue Ice line of products used for cooling ice chests and similar applications.