Aristotle begins by distinguishing two senses of justice. In the broadest sense, justice is the whole of moral virtue as it bears on our treatment of others. The person who is courageous, temperate, and generous is, in this sense, just in their dealings with others — they bring their full character to their relations with other people. This is why justice is sometimes called the complete virtue.
In the narrower sense, justice is a specific virtue concerned with the fair distribution of goods and the correction of unfair transactions. Distributive justice governs how honours, money, and offices should be allocated among the members of a community — according to merit, on Aristotle's view, so that the ratio of shares corresponds to the ratio of relevant merits. Corrective justice governs transactions: when someone gains unfairly at another's expense, justice requires restoring the balance.
Among the most enduring passages in Book V is Aristotle's discussion of equity. Law necessarily speaks in general terms, but every situation is particular. When applying a general rule produces unfair results in a specific case, equity — the correction of the law's excess of generality — is required. Equity is not opposed to justice; it is what justice looks like when it attends to the particulars that general rules cannot anticipate.
Aristotle closes Book V with a subtle question: can a person treat themselves unjustly? His answer is that strictly speaking they cannot — injustice requires two parties, and what looks like self-harm is better understood as a failure of a different virtue (temperance, prudence) rather than of justice. The person who ruins themselves through excess is not unjust to themselves; they are incontinent or intemperate. Justice is irreducibly other-directed.
Justice is the exclusive subject of Book V of the Nicomachean Ethics, one of the most analytically precise sections of Aristotle's ethical writings.