Duality (noun)

1. The state or quality of being twofold; duality.
2. The existence or presentation of two separate and contrasting aspects or qualities.

Origin:

Late 16th century: from french dualité or late latin dualitas, from latin dualis 'double, two each', from duo 'two'.

Examples:

1. The duality of light as both a wave and a particle is a well-known example of quantum mechanics.
2. The duality of good and evil is a common theme in religious and philosophical texts.
3. The duality of the human mind, with its capacity for both reason and emotion, has fascinated psychologists for centuries.
4. The duality of love and hate, peace and war, and life and death are universal experiences that are common to all human beings.
5. The duality of the economic system, with its emphasis on both competition and cooperation, has long been a subject of debate and study.