It's not:

A Location (Cosmo Canyon, North Crater, Shinra Building, Cloud's Villa, Mideel, Temple of the Ancients)
An Enemy Skill (Shadow Flare)
A Monster (Ultima Weapon)
A Limit Break (Great Gospel)
Materia (Black Materia)
Pheonix
Sephiroth
Lucrecia
Meteor
Supernova
Princess Guard
Flayer
Parasol

It's some kind of obtainable item and the last stanza refers to Aeris' sacrifice. That leads me to assume it's Aeris-related, although it just may mean it only is available after Aeris' demise.

Is it the copy of Sepiroth's sword you can get as a consolation prize in the shooting game?