Light is a dimension also called 'Material World', or 'Real World'. It was created at the dawn of time, when The Eldest brought an end of Primordial Chaos, by spliting it into Light and Dark and establishin a boundary between them. In its pure state, Light is a world of matter, biology and physics, essentially magicless and relying purely on science. It is also a world completely devoid of any sentient life, as true sentience - and ability to think beyond biological instincts - is a domain of the Dark. True separation of the two dimensions was never possible - or, to be exact, was not something that would be in any beneficial, as it would mean devolving all sentient beings back into animals. What The Eldest truly did wasn't a full separation, since links between Light and Dark remained still, but rather defining them as something unique and separate as a whole, while small parts of them still acting as an anchors. This 'border' however is by no mean stable or perfect. Light and Dark were separated forcefully and this separation still exists, but there are various ways of changing this fact at least locally, with many of them already 'explored' by civilizations that came after the First Empire. An effect of such merging is a creation of areas called magical anomalies, that are in fact zones where Light and Dark underwent a partial merging, creating an area slightly similar to Primordial Chaos, albeit with Laws of Magicks still in effect (which prevents them from truly changing into the form they had before the time began).
The whole world - including the planet called Gates of Eternity, its moons, local sun, the whole star system and everything in the observable universe and beyond - that can be observed, measured and predicted is Light. If science explains it, then it is light. If it can't, then it isn't.
Fauna & Flora
All creatures that do not require an input of magic (and thus - connection to the Dark) to exist, that weren't created by gods or other beings (Command AI excluded), including flora, fauna, and creatures that escape this categories.
All metals and minerals that were not created by magical enhancement of ores, in short all iron, coal, copper and so on, even if they were created in magical anomalies through Dark, they are still essentially objects of Light. This makes the border especially thin, as many stones, metals and minerals are a fusion of the two in various proportions.