uni means "Sea Urchin" in English.

uni written with kanji is 海胆.
These are jouyou kanji 常用漢字, taught in junior high, chuugakkou 中学校.
Romaji u ni
Kanji 海胆
Meaning Sea Liver
School Level 小2,

uni may also be written as 海栗.

Orthography Notes

  • uni is a jukujikun 熟字訓 reading for 海胆 in this word. That is: those kanji are only read that way when they come together like that.

Homonyms & Similar Words

  • uni 雲丹, written with those kanji, refers to urchin used as an ingredient.

Further Reading

Wikipedia: 雲丹 (Sea_urchin)