What is the meaning of 육계?

cinnamon, cassia (Cinnamomum cassia)

ushnisha, the fleshy swelling or top-knot on the head in depictions of the Buddha.

chicken raised and fattened primarily for their meat

physical world

ṣaḍdhātu, the Six Elements (earth, water, fire, wind, space and consciousness)

the Six Paths, three evil and three good

Source: wiktionary.org