Evil
According to St. Thomas Aquinas, evil is the privation of Good.
late 14c., privacioun, "condition of being without (something);" mid-15c., "act of depriving, act of removing or destroying property;" from Old French privacion and directly from Latin privationem (nominative privatio) "a taking away," noun of action from past-participle stem of privare "to deprive, rob, strip" of anything; "to deliver from" anything (see private (adj.)). Broader meaning "state of being deprived, want of life's comforts or of some necessity" is attested from 1790.
Thus evil is dependant on good to exist, as you cannot be absent something if it is non-existant. This understanding also enables insight into why evil starts small and and is a descent. The descent is an analogy, it is a journey away from light.