Regeneration energy

Regeneration energy was the energy a Time Lord produced as a product of regenerating. A Time Lord typically has enough natural energy to regenerate twelve times which gave them their thirteen lives, (TV: The Deadly Assassin) although the High Council could provide an individual with another cycle of energy should they desire, (TV: The Five Doctors) as in the case with the Master, (TV: The Sound of Drums) and the Doctor. (TV: The Time of the Doctor)

Even after expending their allotted twelve regenerations, a Time Lord still possessed some regeneration energy in their final life, as demonstrated by the Eleventh Doctor. (TV: The Angels Take Manhattan, The Time of the Doctor) As demonstrated by Azmael, a Time Lord could initiate a thirteenth regeneration that would result in their death. (TV: The Twin Dilemma) If a Time Lord died with no full regenerations left, their body could release one last burst of regeneration energy which caused the corpse to vanish altogether, although one had to will this to happen prior to death (PROSE: Shada) and this was never seen to happen to a Time Lord who died because they were killed in a way preventing regeneration, rather than at a point where they had expended all their lives. (TV: The Doctor Falls)

Appearance
It was typically golden in appearance, (TV: The Parting of the Ways, The End of Time, Let's Kill Hitler, The Night of the Doctor, The Day of the Doctor, The Time of the Doctor, Hell Bent, The Doctor Falls, Twice Upon a Time) but it could occasionally be a bright ray of colours instead. (TV: Time and the Rani, Utopia) It could also take the form of electricity. (TV: Doctor Who) In one instance, the energy was visible through the Thirteenth Doctor's skin, on her arms and veins, exhibiting she had two pulses. (TV: The Woman Who Fell to Earth)

It occasionally trickled from the hand as a sign of imminent regeneration, varying from barely noticeable, (TV: The Parting of the Ways, The Stolen Earth, The End of Time, The Doctor Falls, Twice Upon a Time) to clearly visible. (TV: Day of the Moon, Let's Kill Hitler, The Night of the Doctor, The Time of the Doctor, The Doctor Falls, Twice Upon a Time) It also flowed from other body parts, typically the face, head and neck. (TV: The Parting of the Ways, Utopia, The End of Time, The Time of the Doctor, Twice Upon a Time, The Woman Who Fell to Earth, AUDIO: The Brink of Death) Although, at least in the Doctor's case, some regenerations did not yield any noticeable display of regeneration energy, merely transitioning from one body to the next. (TV: Planet of the Spiders, Logopolis, The Caves of Androzani)

Iris Wildthyme's entire body glowed a bit when she regenerated into her Jane Fonda incarnation. (PROSE: The Scarlet Empress) The hermit also emitted a blazing light when he regenerated. (PROSE: The Taking of Planet 5)

Power
The energy itself was quite powerful, typically encouraging those nearby to back away, (TV: The Parting of the Ways, The Stolen Earth, Let's Kill Hitler) and caused significant damage to the Doctor's TARDIS more than once. (TV: The End of Time, Twice Upon a Time) It could also be used as a weapon; River used it to knock down several soldiers after they shot her, (TV: Let's Kill Hitler) and the Eleventh Doctor was able to destroy an entire Dalek fleet when he started regenerating. (TV: The Time of the Doctor) The Tenth Doctor could also use the regeneration energy stored up in a circuit of the TARDIS to disintegrate a platoon of Cybermen. (TV: The Age of Steel) The Eleventh Doctor also claimed his regeneration energy could destroy the Cyber-Planner Mr Clever. (TV: Nightmare in Silver) The energy stored up in Donna Noble's mind was powerful enough to blast several copies of down when triggered by her remembering the Doctor's face, though it knocked out her for several hours as well. (TV: The End of Time)

Pain
The energy also sometimes brought the Time Lord regenerating extreme pain; the Ninth Doctor stumbled back as some surged from his hand, and he continued clutching his torso in pain until he submitted to the process. (TV: The Parting of the Ways) and Mels screamed in agony while purging themselves of the energy, (TV: Utopia, Let's Kill Hitler) and the Eighth Doctor collapsed to the floor, groaning, when his regeneration started. (TV: The Night of the Doctor) The Tenth Doctor briefly collapsed and struggled to walk towards his TARDIS because of the pain. The Eleventh Doctor initially cried out in pain when emerging from the Tenth Doctor's explosive regeneration. (TV: The End of Time) The Twelfth Doctor continuously stumbled and fell while resisting his own regeneration, making pained noises during the process. (TV: Twice Upon a Time)

Residual energy
The Tenth Doctor remarked that the energy was in excess for roughly 15 hours after regenerating, enabling him to restore his hand after it was chopped off. (TV: The Christmas Invasion) River Song was also able to survive being shot by a group of Nazi soldiers due to the excess energy. (TV: Let's Kill Hitler) The Thirteenth Doctor even survived a fall from a great height and crashing through a train's roof, surviving with no noticeable injuries or concussion. (TV: The Woman Who Fell to Earth) It was sometimes harmlessly expelled through the mouth, for a while after regeneration. (TV: The Christmas Invasion, The Eleventh Hour, The Woman Who Fell to Earth) The Twelfth Doctor also remarked that the energy created in the initial beginnings of regeneration could reignite a Time Lord's own life force, placing them in a "state of grace". Although, resisting the process would eventually backfire on them, leading to a severe loss of energy, requiring them to purge themselves of the energy and completely regenerate or continue to resist, leading to death. (TV: Twice Upon a Time)

The energy was capable of creating an entirely new individual based on the Time Lord who produced it. In one instance, the Tenth Doctor used up one of his cycle's regenerations while still keeping the same face by channelling it into his former hand, (TV: Journey's End) which was sliced off by Sycorax leader, (TV: The Christmas Invasion) allowing himself to heal his body, yet avoid physical changes. This regeneration energy remained active and was accidentally interacted with by a human, Donna Noble, triggering a two-way human-Time Lord meta-crisis which caused the hand to regenerate the missing body of its owner, creating a human-Time Lord copy of the Doctor and causing Donna to gain the mind of a Time Lord. However, the presence of two minds inhabiting Donna at once nearly destroyed her and the Doctor was forced to block all memories of him from her consciousness to stop her Time Lord half from remaining active. The Meta-Crisis Doctor also lacked the ability to regenerate. (TV: Journey's End)

The energy could also be conjured for use without the need to regenerate, though doing so was considered inadvisable because it could have repercussions on future regenerations. The Tenth Doctor transferred ten years of his life worth of the energy into a single living circuit in his TARDIS after it lost power when travelling into a new universe, giving it enough power that it could revive the entire dead TARDIS within 24 hours, although it would only be able to operate for five minutes in the new universe before it 'died' again unless returned to its own reality. (TV: Rise of the Cybermen / The Age of Steel) When River Song- currently in her third incarnation- used all of her remaining regeneration energy to bring the Eleventh Doctor back to life after she infected him with a fatal poison, the process nearly killed her and destroyed her ability to regenerate. (TV: Let's Kill Hitler) Even after he had used up all twelve of his regenerations, (TV: The Time of the Doctor) the Eleventh Doctor was able to conjure up enough regeneration energy to heal River's broken wrist. (TV: The Angels Take Manhattan) The Twelfth Doctor willingly expended some regeneration energy to restore Davros' eyesight while claiming it would also cost an arm or a leg down the line. (TV: The Witch's Familiar) His Shadow World counterpart used a device to siphon energy from his personal future to temporarily restore his own eyesight, speculating there may be the potential cost of making all his future regenerations blind, not being able to regenerate again or dying shortly after. (TV: Extremis) The Twelfth Doctor also conjured a small portion of the energy to prove to his first incarnation that they were the same person. (TV: Twice Upon a Time)

As a food source
The alien race known as the Ravenous essentially evolved to 'feed' on regenerative energy, making them the predator of the Time Lords; although they could also feed on humans, the Doctor compared this to humans settling for a packet of crisps before moving on to the main 'steak dinner' of a Time Lord (AUDIO: Seizure, Deeptime Frontier). However, Time Lords suffering from regenerative dissonance were essentially immune to the Ravenous, as their condition 'contaminated' the regenerative energy to make it unappealing to the Ravenous. (AUDIO: The Odds Against)