Glaze aircraft icing (also known as clear icing) is hard and transparent. It is formed by the relatively slow freezing of supercooled water droplets as they spread over the surface. It tends to acccumulate more rapidly than rime icing, and is often very hard and therefore more difficult to remove. Since it does not freeze instantly, the ice can form into shapes that cause significant aerdynamic penalties, and it therefore the most hazardous form of icing. However, it is the least frequent type of ice encountered, reponsible for about 10% of icing reports.
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use