Quintessence is light in its purest form. It's very powerful and can be used to leap to specific places. It is used to make frissyn, the only substance able to stop Everblaze. It was also mentioned that its alchemy symbol is a Q with an X going through it.
Acquiring Quintessence
Quintessence can only be acquired from unmapped stars, and it's illegal to bottle it. You can bottle it using a stellarscope, like any other star; however, the scope gets extremely hot, as Sophie discovered in the first book.
You can bottle Quintessence from the stars Elementine, Lucilliant, Phosforien, Marquiseire, and Candesia.