The Veil

The Veil is a metaphysical barrier that exists between the physical Thedas and the Fade. Neither spirits nor mortal beings can easily pass physically through the Veil, but the consciousnesses of most mortal dreamers and all Mages are a different matter (though the average dreamer is unlikely to ever be aware of such 'travels'). The Veil is not an object − it cannot be touched or seen. It is not a physical barrier, but rather something similar to a magical vibration that repels the Fade. It is a metaphor used by mortal scholars to explain the interactions between the two different realities.

Historical Basis

There is no mention of a time when the Veil didn't exist in human history

Variations & Mutation

Tears:  It is theorized that the Veil tends to be weaker in places that feature, or have experienced, extensive death or use of magic. The former occurs because spirits are attracted to death and they press against the Veil. The Veil is particularly thin at night, when most people sleep and spirits are most active. The use of blood magic can allow the Veil to be torn so that demons may physically pass through it into the waking world. 
Weakened Veil can be recognized by its characteristic green hue. The Dalish call places where the Veil is thin "setheneran" − a land of waking dreams. Such areas are a habitat for giant spiders and the rare plant felandaris, also known as "demon vine". Areas where the Veil rips open are usually referred to as "tears" in the Veil, in keeping with the fabric metaphor. In fact, when the Veil itself tears it makes a sound like ripping cloth.
"Mending" these Tears, i.e. strengthening the Veil, is extremely difficult. Depending on the level of damage done to the Veil, it can be closed through several methods. For example, in Ostagar,The Warden-Commander was able to mend several Veil Tears by killing powerful spirits or demons keeping the Veil open.