Samantha's alternate timeline diverged from the one we know in 1955. Magic, which had sunk below the Earth's crust in centuries past, began once more to flow across the surface of the planet. This event, called the Great Surge, sparked a devastating world war.
I have made every effort to avoid spoilers here; what follows is a simple definition of terms and a collection of lore.
Magic: Supernatural force that has an effect on the physical world. Harnessed by mages, usually through song.
Blood Magic: Songs which are performed by utilizing magic harvested from Source or Snap blood.
Mage: Any creature capable of both perceiving and using magic.
Current: A strong directional flow of magic. Sometimes used interchangeably with "melody." Slang developed due to the similarities between magic and both water and electricity.
Melody: The ambient sound of magic in a specific area. This may also refer to the sound of a person or object, if the subject is magical in nature or enchanted.
Song: Slang term for a spell. The most common way for mages to utilize magic. Lyrics are not required.
Surge: An increase in the strength of a specific current in a localized area. Most common at places where the current emerges from the ground.
Upwell: A place where the current emerges from the ground.
Spring: Same as upwell.
Source: A location or creature that generates more magic than required to maintain its existence. Upwells are sources, as are awoken animals and, to a lesser degree, mages.
Snap: An entity of human origin that generates enough personal magic to cause physical changes. Those changes typically happen at puberty, although may be evident at birth if both parents were snaps of the same species.
Latent: A human Source that did not "snap" into something else at puberty.
Awoken animal: An animal Source. Their ambient magic gives them human-like intelligence.
Milking: The act of draining blood from a living Source or Snap. In most cases, this is both illegal and involuntary.