"Una furtiva lagrima" (A furtive tear) is the romanza from act 2, scene 8 of the Italian opera L'elisir d'amore by Gaetano Donizetti.
It is sung by Nemorino (tenor) when it appears that the love potion he bought to win the heart of his dream lady, Adina, is working.
He loves Adina, but she is not interested in an innocent, rustic man such as he.
Ponziano Loverini | Portrait of Gaetano Doninzetti, 1877