duality; dual nature
Refers to the state of having two contrasting aspects, natures, or principles within a single entity. Often used in philosophical, psychological, or descriptive contexts.
There is a duality of good and evil in the human heart.
This substance exhibits wave-particle duality.
二面性 means 'two-facedness' or 'double-sidedness', often with a nuance of inconsistency or hypocrisy, whereas 二重性 is more neutral and abstract.
Compound of 二重 (にじゅう, 'double') + 性 (せい, '-ness, nature'). The exact historical derivation is uncertain, but the word follows a common pattern for abstract nouns in modern Japanese.