柳 is the Chinese, old Korean Hanja, and Japanese Kanji for willow (as in willow tree).

In ancient Chinese astronomy/astrology, this can refer to the “Willow” constellation (one of the 28 mansions).

If you want to get scientific, this can refer to any tree of the genus Salix including the weeping willow (Salix babylonica).

Notes:
In Chinese, this can be the surname Liu.
In Japanese, it can be the surname Yanagi. It can also be pronounced other ways for different rare surnames. Some Japanese use this as a female given name.
In Korean, this can be the surname Yu, though most of the time it’s pronounced ryu in Korean.

花柳界 literally means “Flower Willow World/Kingdom.”

In Japanese, this means “The Realm of the Geisha” or “World of the Geisha.” I suppose there is a presumption that the Geisha is surrounded by flowers in their residence. In Chinese and Korean, this pretty much has colloquially come to mean “The Red Light District” or to refer to pimps, prostitutes, and johns as a group.

This is a common Mandarin Chinese transliteration for the male Polish name Mariusz.

馬特利柳斯 is the name Martrelius in Chinese (Mandarin).

This is Tao Yuanming's poem, “Returning to Dwell in Gardens and Fields.”

少無適俗韻 性本愛丘山。
誤落塵網中 一去三十年。

羈鳥戀舊林 池魚思故淵。
開荒南野際 抱拙歸園田。

方宅十餘畝 草屋八九間。
榆柳蔭後簷 桃李羅堂前。

暖暖遠人村 依依墟裡煙。
狗吠深巷中 雞鳴桑樹顛。

戶庭無塵雜 虛室有餘閒。
久在樊籠裡 復得返自然。