Rime ice

A rough ice that forms on aircraft flying through visible moisture, such as a cloud, when the temperature is below freezing. Rime ice disturbs the smooth airflow as well as adding weight.

Aviation Maintenance Technician Handbook — Airframe, p. G-29

Also defined in 1 more source

Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.

Pilot's Handbook of Aeronautical Knowledge, p. G-26