Full Idea
My general description of injustice is this: the mastery of the soul by anger, fear, pleasure, pain, envy and desires, whether they lead to actual damage or not.
Clarification
'Justice' is the Greek word 'dikaiosuné', which is also general moral rightness
Gist of Idea
Injustice is the mastery of the soul by bad feelings, even if they do not lead to harm
Source
Plato (The Laws [c.349 BCE], 863e)
Book Reference
Plato: 'The Laws', ed/tr. Saunders,Trevor J [Penguin 1970], p.374