Avatar State

"The Avatar State is a defense mechanism, designed to empower you with the skills and knowledge of all the past Avatars. The glow is the combination of all your past lives, focusing their energy through your body. In the Avatar State, you are at your most powerful, but you are also at your most vulnerable. If you are killed in the Avatar State, the reincarnation cycle will be broken and the Avatar will cease to exist." — Roku to Aang in "The Avatar State".

The strongest and most powerful ability that an Avatar can invoke is the Avatar State. It allows an Avatar to channel immensely vast cosmic energies and knowledge of all of the previous Avatars, enabling them to perform especially powerful and amazing bending feats. Avatars may unconsciously enter such a state in response to perception of mortal danger, utilizing the state as a defense mechanism, or emotional distress. While in this state, an Avatar has access to bending techniques they themselves may have yet to learn but has no conscious control and likely causes great collateral damage. Avatars with mastery of the Avatar State have full conscious control over the state and can enter and exit it at will. Avatars with mastery have eyes that glow momentarily, not continuously. It was by entering the Avatar State defensively that Aang was able to freeze himself and Appa inside an iceberg for a hundred years by using a combination of airbending and waterbending. He did not physically age at all during the century he was in suspended animation, but it would take its toll on him to be in the Avatar State for a Hundred Years.

While in the Avatar State, the Avatar's eyes glow, as do any body markings following Chi paths such as Air Nomad tattoos; however, if the body has received an injury deeper than the tattoo, then that spot will not glow in the state. The glow is the combination of all the past lives, focusing their energy through the body. When Aang spoke in the Avatar State, his voice encompassed a combination of several different voices, presumed to belong to past Avatars, at once

Avatar_line-up.png the Avatar State works
There seems to be two distinct ways to how the Avatar State works:

Continuous
When the State works continuously, the Avatar's eyes, and tattoos in the case of an Air Nomad Avatar, glow white constantly, and the spirits of the past Avatars actively channel their energy, skills, and knowledge through the body of the current Avatar, granting him or her great amounts of strength and power.

The State has been seen to work this way almost exclusively as instinctive unconscious reaction, as the 'defense mechanism' Roku described the State as, when an Avatar is confronted with a threat beyond that they can deal with, or when their emotions, specifically rage and sorrow, reach a breaking point. So it may perhaps be more accurate to describe the State being used this way as 'reflexively' rather than 'continuously'. When the State is triggered this way the combined spirits of the past Avatars seem to take control of the Avatar's body and voice, thus the current Avatar has little to no control over his own actions and may retain little to no memory of what he did, though this is not always the case.

Aang appeared to use the State this way against Fire Lord Ozai after the scar on his back was forced against a rock unlocking his final chakra. Just as Guru Pathik had said, this event led to Aang being able to control the State himself.

Unlike previous times where Aang entered the State, he was not physically exhausted. This is possibly due to unlocking the final chakra, which let him regain control over the collective spirit.

Non-continuous
When the State is used non-continuously, the Avatar's eyes, and tattoos in the case of an Air Nomad Avatar, glow only for a second, and during that time the current Avatar stores the knowledge and energy needed within himself so that he can release it once he exits the state, taking only the power needed for a specific task. This reduces the danger of being killed in the Avatar State and enables a more willful, controlled use of the Avatar State's power. This can only be done by an Avatar who has mastered the State as it is a conscious use rather than an instinctual response. Mastery of this technique is part of what distinguishes an Avatar as "Fully Realized".

Spiritual link-up
The Avatar State also allows an Avatar to join with certain spirits. This was demonstrated when Aang entered the Avatar State in order to aid the frantic Ocean Spirit. Both Aang in his Avatar State and the Ocean Spirit combined their abilities to defend the Northern Water Tribe after all waterbenders were rendered defenseless. All Avatars can make these contacts by spiritually calling to different spirits.

Weakness
While the Avatar is all but invincible in the Avatar State, having millions of bending experiences, there is one grave weakness - while in effect, every Avatar, or the entirety of the earth's spirit, is present in the physical world while the current Avatar is in the Avatar State. If the Avatar were to be killed while in the Avatar State, every Avatar would die with them. This would break the cycle permanently and the Avatar would cease to exist.

Aang was fatally wounded by a lightning strike from Azula while entering the Avatar State. As a result, his connection with the Avatar Spirit was jeopardized, resulting in the Avatar Spirit's fall from the plane of existence. Fortunately, Katara was able to use the Spirit Water she received from Pakku to heal Aang, reviving Aang and consequently the Avatar Spirit. The strain of the revival caused Aang to slip into a coma for several weeks, and during that time, he managed to secure his connection to the Avatar State by meeting with the four Avatar incarnations before him. However, it took longer for his connection to his past lives to be completely restored.

Another downside to the Avatar State is the considerable mental strain it can cause to less experienced Avatars. Prior to his tutelage under Guru Pathik, Aang would often leave the Avatar State feeling horrified at what he had done under its influence. Furthermore, use of the state, at least in extraordinary circumstances, can place such a large physical strain on an Avatar's body such that their life span can be affected; thus, Aang's one hundred-year time in the iceberg contributed greatly to his eventual death at sixty-six years of age biologically, and 166 years chronologically.