(physics) the property of matter and electromagnetic radiation that is characterized by the fact that some properties can be explained best by wave theory and others by particle theory
"The experiment highlighted the duality of light, showing how its behavior shifts between acting like a spreading wave and a distinct particle depending on what we measure."
(geometry) the interchangeability of the roles of points and planes in the theorems of projective geometry
"In his lecture on projective geometry, the professor illustrated duality by showing how a theorem about collinear points transforms seamlessly into one about concurrent planes when their roles are swapped."
A classification into two subclasses or opposed parts.
"The new tax code's duality divides income sources into labor earnings and investment gains for separate calculation methods."