A period of time defining a gradual change in the way magic operates in the world.
Prior to this period, magic was extremely difficult and required tremendous resources to accomplish even trivial effects. Years of study and preparation and thousands of platinum of gold would go into a spell to enchant a sword with a sharper edge. Magic was weak, but it was safe. It accomplished what it set out to predictably and formulaically.
By the end of this period, magic had become significantly easier to cast, to the point where anyone could manage to learn it with some effort. Spells could be cast by those who knew how with only a token effort. Magic had become strong, but dangerous. Spellcasters and those around them started to become Manawracked.
The power structures that had existed prior to this period broke down, as in the middle magic had reached a perfect balance of safety and usability to allow those who embraced it to affect enormous change in the world at their will.
By the end of this period, the world had changed, not only in structure but also in culture. Where before magic-users were revered as devoted scholars or the most pure of belief, after they were seen as dangers to their community and hunted.
While theories abound regarding exactly what caused the chance in magic, none of them have ever been proven. People have adapted to live in the new world that the change in magic left behind. Some have made peace with it, while others word to discover why and reverse the change. Others still work to push the change further and give magic even more power.