Thaumogenesis is the process through which a new being is created as a by-product of a spell.
Appearances
Superstar
Jonathan Levinson perfromed an augmentation spell to create an alternate universe in which he was the best at everything. This spell had the unfortunate side effect of creating a humaniod demon which when killed, broke the spell and reverted the world to it's natural state. The word "thaumogenesis" is never used, but the concept was in effect.
After Life
After Willow perfromed a spell to raise Buffy from the dead, a demon was created that possesed Anya, Dawn and Xander briefly, it also briefly created a copy of Buffy that it possesed and altered Buffy's perception of some photographs. Willow and Tara perfromed a spell to make the demon corporial, so that Buffy could fight and kill it. This is the first actual mention of thaumogenesis
The Price
Angel and Lilah perormed a dark spell in the Hyperion Hotel lobby , to open a tear in reality to enter Quor Toth. They believed that it had failed and when some slucks came out of it they surmised that they demons were created via thaumogenesis. When other demons and people began emerging from the rift, they realised that thaumogenesis was not at play.