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Regeneration was the process of "molecular readjustment" by which Time Lords and others renewed themselves, causing a complete physical and often psychological change. It could happen because of severe illness, old age, or injury. It could also be invoked by choice, whether voluntary or involuntary.
Conversely, regeneration could be prevented by choice, although choosing not to regenerate from fatal damage was essentially an act of suicide. However, a Time Lord could delay the actual change in appearance and enter "a state of grace" for a brief period of time.
The Tecteun was one of the first inhabitants of Gallifrey to explore other planets. She discovered the Timeless Child on another planet, and adopted her as her own. She later brought the child back with her to Gallifrey. When Tecteun discovered the child's ability to regenerate, she experimented on several of their incarnations to discover the secret to this ability.
Eventually she injected herself with the genes which had been spliced from the child. Upon doing so she immediately regenerated into a male incarnation. Tecteun later gave his people this gene so that they would be able to regenerate, the species quickly worked out time travel and rechristened themselves the "Time Lords".
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Time Lords released massive amounts of a hormone called lindos in moments of extreme trauma, and it was this hormone which triggered regeneration. Newly regenerated Time Lords could be identified by elevated levels of lindos in their system.
During regeneration, a Time Lord experienced a surge of pure regenerative energy, as their entire system was rewritten and their universe moulded into a new shape, with their very biodata being rewritten in the fabric of space-time.
During a regeneration, a Time Lord's body could shine with milky white light, a swirl of rainbow colours or no colours, crackle with electricity, (appear to be engulfed in flames, or discharge golden energy, which could vary in speed and intensity from being brief and contained to explosive and capable, in extreme circumstances, of causing damage to the nearby area. In other cases, there was no apparent energy discharge at all, just a fade away to the next incarnation.
The Time Lord would change in appearance, height, mass or apparent age. The personality would also change; even the cells and chemistry of the brain regenerated, although their blood type would remain the same in all their lives.
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Regeneration mixed up the synapses in the brain, sometimes resulting in negative after effects. During the first few hours of the regeneration, the Time Lord could suffer from confusion, erratic behaviour, extended periods of unconsciousness, or memory loss. Motor control could be impaired and a Time Lord could suffer random spasms as the regeneration settled.
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Time Lords usually had a limited cycle: twelve regenerations, consisting of thirteen incarnations, after which they would suffer permanent death. As with most "rules", there were occasional exceptions, and the High Council was capable of offering Time Lords a new lease of life in the form of a new regeneration cycle.
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Bi-generation was a rare mutation of regeneration that was believed by the Doctor to be merely a myth. After being mortally wounded, instead of changing directly into the new incarnation, the new incarnation would instead split from their previous self, existing as a separate entity while the original incarnation involved would be healed of their fatal injuries.
After being mortally wounded by the Toymaker, the Fourteenth Doctor underwent a bi-generation instead of a regular regeneration. The regeneration at first appeared to be normal, but when the regeneration energy disappeared, the Fourteenth Doctor remained himself, although he felt something different. At his request, Donna Noble and Melanie Bush each pulled on one of the Fourteenth Doctor's arms, causing the Fifteenth Doctor to begin emerging from his body. The two Doctors then finished the bi-generation by shoving each other away from themselves.