Abismo translates to "abyss" or "chasm". It's a dramatic and evocative word that immediately brings to mind a deep, impassable divide.
In "Por Ti Volaré", Andrea Bocelli uses abismo to describe the profound separation between himself and his beloved. He sings, "Separado del mío por un abismo" (Separated from mine by an abyss), powerfully conveying the immense distance and challenges he feels in reaching her world. This word perfectly captures the song's theme of overcoming vast obstacles for love.
English speakers usually meet this melody as "Time to Say Goodbye", which rather misses what the Spanish version is doing. Nobody is leaving here. The singer is trying to get to someone. He opens alone, dreaming of "un horizonte falto de palabras" (a horizon short of words), and tells us "En la sombra y entre luces todo es negro para mi mirada" (in the shadow and among lights everything is black to my eyes) when she is not beside him. She has a world of her own, "Separado del mío por un abismo" (separated from mine by a chasm), and a chasm is why the answer has to be flight rather than a walk.
Then the title arrives, and keeps arriving. "Por ti volaré" (for you I will fly) repeats until it stops sounding like a promise and starts sounding like an itinerary, route included: "Por cielos y mares hasta tu amor" (over skies and seas to your love). The best bit is quieter than the big ending, though. In the second verse he describes "Una luna hecha para mí" (a moon made for me), lit for him and nobody else, and he still sounds lonely about it. A moon of your own is not much use when the person you want is on the other side of the water.