Quintessence is a substance with the highest known energy per unit volume in the universe. Raw quintessence is capable of being extracted from planets and refined into a standardized fuel for Galra technology.
Quintessence is seen in various forms:
- Yellow: Raw quintessence. The Druids take this and convert it into purple quintessence, as seen in Season 1 Episode 10 - 'Collection and Extraction'.
- Purple: Standard fuel for Galra technology. When injected into living beings such as Haggar's Robeasts, the Beta Traz Warden, and later on Zarkon through his armored suit, it causes massive increase in a subject's size, strength, and speed, at the potential cost of their sanity.
- White-Blue: Concentrated quintessence. Lotor uses this to try and travel through the inter-reality gate that he had constructed over the dimensional rift left by the landing of the trans-reality comet. It is implied to be very powerful, as it overloads Kolivan's monitor in season 4 episode 1 'Code of Honor'. It is later discovered in season 6 episode 4 - 'The Colony' that some of this form of quintessence is harvested by Lotor from living Alteans.
It is heavily implied that Zarkon's unnaturally long lifespan as well as his immense physical strength is due to regular infusions of refined quintessence. His change of personality from honorable Paladin to monstrous dictator is also due to the influence of quintessence, which exacerbates an individual's worst traits and can drive them to insanity.
Throughout the first season of Voltron: Legendary Defender, we see multiple references to the substance. The most common example is Zarkon's ability to levitate between numerous spires that blast the energy at him, and it is hinted at as an explanation for his longevity.
The Galra Empire takes over planets in order to harvest quintessence, and they find that the crystals that Balmera naturally produce can contain quintessence. The Galra use mining shafts to dig deep into the Balmera and harvest the crystals. Unfortunately, this leaves no room for the Balmera to gain something in return, and thus, the only seen Balmera, X-95-Vox, is severely damaged when the Galra mine it.
Later in the first season, Haggar creates a massive device called the Komar Experiment that harvests all possible quintessence from a planet, leaving it completely barren of any life. Zarkon remarks that this will allow them to quickly gain massive amounts of quintessence, which would normally have taken years with mining and colonization.
- The idea of Zarkon extending his lifespan with substances was touched upon in the original Voltron series, where Zarkon briefly mentioned using the flowers from the planet Lyra to extend his already long life. 
- Per Wiktionary: Etymology: Middle English, from Middle French, from Medieval Latin quinta essentia "fifth essence".
- Noun: quintessence (countable and uncountable, plural quintessences)
- A thing that is the most perfect example of its type; the most perfect embodiment of something; epitome, prototype.
- A pure substance.
- The essence of a thing in its purest and most concentrated form.
- (alchemy) The fifth alchemical element, or essence, after earth, air, fire, and water.
- (physics) A hypothetical form of dark energy postulated to explain observations of an accelerating universe.
- Infusion with a living being seems to cause emaciation and gradual insanity of the user, as Honerva and Kova seem drastically lose weight and suffer radical changes in personality.