This is the 9441st most frequent Japanese word.
空洞
"空洞" means "hollow" or "cavity."
空洞 is used to describe a physical hollow or void within an object.
木の幹に空洞がある。
There is a hollow in the tree trunk.
空洞 is metaphorically used to suggest a lack of substance or sincerity.
彼の言葉には空洞が感じられる。
His words feel hollow.
空洞 is applied in a scientific or conceptual context to refer to empty spaces.