一个 + 空洞 / 空洞 + 的 + Noun
The adjective 空洞 (kōngdòng) means "hollow," "empty," or "void." It can describe both physical emptiness (a hollow space) and abstract emptiness (hollow words). As a noun, 一个空洞 means "a hole" or "a void."
他的演讲内容很空洞,没什么实际意义。
Tā de yǎnjiǎng nèiróng hěn kōngdòng, méi shénme shíjì yìyì.
His speech was very empty and had no real meaning.
科学家在地下发现了一个巨大的空洞。
Kēxuéjiā zài dìxià fāxiàn le yī gè jùdà de kōngdòng.
Scientists discovered a huge cavity underground.
光说空洞的口号没有用,要拿出行动来。
Guāng shuō kōngdòng de kǒuhào méiyǒu yòng, yào náchū xíngdòng lái.
Just shouting empty slogans is useless; you need to take action.