noun
Gelidium japonicum (species of seaweed)
A specific type of red algae used in agar production. The name is usually written in katakana.
オニクサは寒天の原料として利用される。
Gelidium japonicum is used as a raw material for agar.
The name likely derives from 鬼 (oni, 'demon') + 草 (kusa, 'grass/plant'), but the exact origin is uncertain.