If you are already familiar with the show, then just look at the first two :)
If you see anything missing that could be added or corrected, let me know!

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Ichor: An ancient bloodline in the Fire Nation, bound to protect the Firelord and the royal family. Through their bloodline is passed a rare form of bending, nearly unheard of; Energybending. Once a revered lineage, the Ichor faded into legend after the last known energybender served Firelord Sozin. Generations later, Ara is born as the prophesied "last hope of the Ichor".

Energybending: A unique ability found nearly solely within the Ichor bloodline, allowing the bender to absorb fire's energy, using it to heal or strengthen themselves. Unlike firebending, energybenders cannot create nor manipulate fire itself; instead, it offers protection against the flame. The full extent of energybenders is greatly unknown, as there have never been an extensive number of them, and what little they did know has been lost to time.
     The Avatar can also do something similar, though perhaps not so specific with fire. He can take away another person's bending abilities by manipulating the energy within ones body.

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Air Nomads: One of the Four Nations in the World, the Air Nomads are known for their peaceful ways and spiritual focus. Air Nomads practice airbending and lived in four temples across the world until the Air Nomad Genocide. (most if not all Air Nomads are airbenders)

Airbending: The ability to manipulate and control air currents, practiced by the Air Nomads. It emphasizes agility and freedom.

Avatar: A unique individual capable of bending all four elements (air, water, earth, fire). The avatar is tasked with maintaining balance in the world, reincarnating in a new nation after death in the order of the elements.

Avatar State: A powerful and dangerous state in which the Avatar can access the knowledge and power of all previous Avatars, enhancing their abilities significantly. It is a great asset and a vulnerable point for the Avatar. If they are killed while in the Avatar State, the cycle of reincarnation will be broken, and the Avatar will be no more.

Ba Sing Se: The massive, fortified Capital city of the Earth Kingdom, known for its thick walls and being almost impenetrable to invasion. It has a complex political system and an undercurrent of secrecy.

Bending: The ability to manipulate one of the four elements—water, earth, fire, or air. Bending is usually inherited by people in specific nations and can be honed with training and practice. Some bending styles have advanced forms, like metal-bending and lightning-bending and blood-bending.

Chi: The internal energy or life force in all living things. Benders use their chi to manipulate elements, and certain bending styles (such as waterbending healing) directly affect chi. Chi can be blocked to prevent bending abilities temporarily.

Earth Kingdom: One of the Four Nations, the Earth Kingdom is the largest in the world and populated by earthbenders (though not all citizens of the Earth Kingdom are earthbenders). It is known for its vast landscapes and diverse cultures within its cities and towns.

Earthbending: The ability to manipulate earth and stone, practiced by earthbenders. It emphasizes strength, stability, and endurance.

Fire Nation: One of the Four Nations, the Fire Nation is a powerful, militaristic empire. Known for its ambition, the Fire Nation was responsible for starting the Hundred Year War in pursuit of global conquest. Populated by firebenders (though not all citizens are firebenders).

Firebending: The ability to manipulate and create fire, practiced by the Fire Nation. It is fueled by inner strength and can be enhanced by emotions, especially anger and determination. Advanced firebending can include blue firebending and lightning generation, known as "lightning bending". To many, firebending is seen as destructive and dangerous.

Four Nations: The main political and cultural divisions in the world, consisting of the Air Nomads, Earth Kingdom, Fire Nation, and Water Tribes, each with its own bending element.

Kyoshi Warriors: A group of non-bending female warriors from Kyoshi Island who use techniques inspired by Avatar Kyoshi, one of the previous Avatars. They are skilled fighters known for their discipline and strength.

Lightning Bending: An advanced firebending technique that allows a firebender to generate and control lightning. It requires intense concentration and mastery of firebending.

Lion Turtle: Ancient, mystical creatures in the world of Avatar, said to be the original source of bending abilities. The lion turtles were once protectors of humanity and gave humans the power to bend the elements.

Metalbending: A specialized form of Earthbending, discovered by Toph Beifong, allowing earthbenders to manipulate metal by sensing the small particles of earth within it.

Order of the White Lotus: A secret society of highly skilled individuals from all nations who share knowledge and culture, seeing to maintain peace and balance in the world.

Southern Water Tribe: A small, resilient community in the South Pole, part of the Water Tribes. Known for its strong connection to the spirits and waterbending tradition. The tribe was impacted heavily by the Fire Nation's raids.

Spirit World: A parallel realm inhabited by spirits and ancient beings. The Avatar has a unique connection to the Spirit World and can enter it to communicate with spirits, seek wisdom, and maintain balance between the physical and spiritual worlds.

The Great Divide: A massive canyon in the Earth Kingdom, considered one of the natural wonders of the world. It is known for its difficult terrain and the divided communities that live nearby.

The Hundred Year War: A global conflict started by Fire Lord Sozin, in which the Fire Nation attempted to conquer the other nations. The war ended with the defeat of Fire Lord Ozai by Avatar Aang.

Waterbending: The ability to manipulate water, practiced by waterbenders from the Water Tribes. It is based on fluidity and adaptability, with advanced forms allowing for healing and even bloodbending.

Water Tribes: One of the Four Nations, consisting of two main communities at the North and South Poles. Waterbenders use water for various purposes, including healing. The Water Tribes are culturally connected to the moon and the ocean spirits.