An example would be that the concept of 'light' necessitates an opposite: 'dark.' The idea of one entails the other. Is there a term for this?
Antithesis is a counter-proposition, necessitating a proposition.
I would either call it dialectical (or, under certain circumstances, controversial).
According to http://www.thefreedictionary.com/dialectical, "dialectic" is:
The process especially associated with Hegel of arriving at the truth by stating a thesis, developing a contradictory antithesis, and combining and resolving them into a coherent synthesis.
I think I would go with quality as the good and the bad are a matter of quality.
One of the Tao texts i have places this under Qualities but another has it under Self-Manifestation through Contraries.
antithesis and dialectical don't really fit as what this is a property of human perception which need not be phrased in terms of logical propositions.
note well that what is called the unity of opposites is a fallacy because good can exist with evil but we can't recognize the good without knowing the evil and vise versa so i offer the qualification that it's the perceptive opposites that are unified.