Quintessence is an overall term for the fifth element. No, not boron; the fifth element in a classical four-element system.
In Fullmetal Alchemist, the most fundamental law of alchemy is Equivalent Exchange; like most real physical laws it’s not always phrased the same, but the basic idea is “To gain something, something of equal value must be given”.
The most-heard phrasing is from the first anime and seems excessively human-centric: “Humankind cannot gain anything without first giving something in return. To obtain, something of equal value must be lost. That is alchemy’s first law of Equivalent Exchange.”
In alchemy that does not involve living beings, this is typically seen as a conservation law; elements are only rearranged, not created or destroyed.