Alimony is a U.S. term denoting a legal obligation to provide financial support to one's spouse from the other spouse after marital separation or from the ex-spouse upon divorce. It is established by divorce

Divorce

Divorce is the final termination of a marital union, canceling the legal duties and responsibilities of marriage and dissolving the bonds of matrimony between the parties...

 law

Law

Law is a system of rules and guidelines which are enforced through social institutions to govern behavior, wherever possible. It shapes politics, economics and society in numerous ways and serves as a social mediator of relations between people. Contract law regulates everything from buying a bus...

 or family law

Family law

Family law is an area of the law that deals with family-related issues and domestic relations including:*the nature of marriage, civil unions, and domestic partnerships;...

 in many countries and is based on the premise that both spouses in theory have a legal obligation to support each other during their marriage

Marriage

Marriage is a social union or legal contract between people that creates kinship. It is an institution in which interpersonal relationships, usually intimate and sexual, are acknowledged in a variety of ways, depending on the culture or subculture in which it is found...

 (or civil union) or upon separation or/and divorce.
Alimony has been discussed in ancient legal texts including the Babylonian Code of Hammurabi

Code of Hammurabi

The Code of Hammurabi is a well-preserved Babylonian law code, dating to ca. 1780 BC . It is one of the oldest deciphered writings of significant length in the world. The sixth Babylonian king, Hammurabi, enacted the code, and partial copies exist on a human-sized stone stele and various clay...

 (#137-#142) and the Code of Justinian.