A lemon was a sour citrus fruit native to Earth.

Jean-Luc Picard offered Simon Tarses lemon for his tea, but he declined. Admiral Norah Satie drank her tea with lemon. (TNG: "The Drumhead")

In 2370, Benjamin Sisko requested a Chiraltan tea with a double twist of lemon. (DS9: "Second Sight")

Lemons are also used to make lemonade. (VOY: "Caretaker", "Future's End")

In 2366, Captain Picard hoped the Barzan wormhole wouldn't turn out as the "proverbial lemon". (TNG: "The Price")