From Destinypedia, the Destiny wiki
Paracausality is a category used to refer to abilities, events or entities which seem to violate causality (i.e. the principle that all effects must have a preceding cause). It is also commonly referred to as "magic".
- "Once upon a time,* a gardener and a winnower lived** together in a garden.***
* It was once before a time, because time had not yet begun.
** We did not live. We existed as principles of ontological dynamics that emerged from mathematical structures, as bodiless and inevitable as the primes.
*** It was the field of possibility that prefigured existence."
- — Unveiling: Gardener and Winnower
According to the Lore Book "Unveiling," which is seemingly narrated by the Darkness itself, both the Light and the Darkness have existed since before the start of time, and thus before the universe came into existence. Being unbound by the laws of causality, neither the Light or Darkness can be said to have a beginning. Rather, they are emergent properties of even more fundamental "mathematical structures" that in turn underlay reality itself. 
In the allegorical story recounted in the Lore Book, the Darkness and Light occupied themselves by engaging one another in a game, analogous to a vastly more complex version of the "Game of Life" devised by Earth mathematician John Conway. This game would consistently arrive at an end-state dominated by a single, self-perpetuating pattern, which subsumed all others in the game. The Darkness found this to be a pleasing outcome, but the Light felt the pattern to be boring, and desired that the game endlessly produce novel patterns instead. In order to promote this novelty, the Light transformed itself into a new "rule" within the game; the Darkness did the same, to counteract the Light's efforts. These new rules were "set aside" from the other rules of the game, so that they could not be limited or influenced by them, but could manipulate the other rules to bring about their intended outcomes.
While the story above is laden with metaphor and open to interpretation, it appears to suggest that the universe is the "game" which the Light and Darkness played in the garden, that the rules of the game are the laws of physics as they are commonly understood, and that the "new rules" that the Light and Darkness became are the basis of paracausality.
- The Veil
- Throughout the game, there has been an established difference between Paracausal and Acausal. Para- is a prefix meaning "beside; adjacent to", while causal refers to the physics of cause and effect. Paracausal is not the same as acausal, which is the opposite of causality with the Greek prefix a- ("not, without"). Paracausal thus means the Light and the Darkness obey physics which ignore or sidestep conventional cause-and-effect systems.