L'illuso translates to "the deluded one" or "the dreamer." It's a less common word that carries a strong emotional weight, suggesting someone who holds onto false hopes or unrealistic beliefs.
In the context of "Ritornerai," the singer uses this word to describe himself, admitting that he has remained "l'illuso di sempre" (the deluded one as always). This self-awareness adds a poignant layer to the song's theme of a hopeful, yet perhaps naive, expectation of a lover's return.