Meaning, literally, “two things simultaneously”.
This is not the same thing as dualism, which implies perception through opposites (i.e., dualism is the human propensity for perceiving black/white, good/evil, god/devil, male/female, etc) Duality implies the evolving perception which enables us to see that past and future, just for example, are no different, but only different perceptions according to our location in time.
Duality further allows for two seemingly contradictory conditions to exist simultaneously, without either obliterating or in any way usurping the other.
Example: “a love/hate relationship”.
Another example: We exist as mortal human beings in the Now, and simultaneously as eternal beings through the energy body of the double.
Duality can be studied in the statement, “You must be immortal before you will know how to become immortal.”
As long as we are locked into a linear, static perception of reality, we are prisoners of dualism.