Evil is Corruption of Measure, Form, or Order
When accordingly it is inquired, whence is evil, it must first be inquired, what is evil, which is nothing else than corruption, either of the measure, or the form, or the order, that belong to nature. Nature therefore which has been corrupted, is called evil, for assuredly when incorrupt it is good; but even when corrupt, so far as it is nature it is good, so far as it is corrupted it is evil.
Source: Concerning the Nature of Good, Against the Manichaeans (New Advent)