Alimony is support (almost always money) paid by one spouse to the other. If you see references to “spousal support”, that’s alimony. You may occasionally see older references to “alimony for the support of minor children”, but generally that term has fallen into disuse in favor of the simpler, more modern phrase, “child support”.
A more analytical definition of alimony is that it is the allowance out of the FUTURE estate of one spouse, for the support of the other spouse. This is an important concept because it is contrasted with property division, which is an allowance out of the CURRENT (marital) estate.
Historically, alimony was awarded as a recognition that the traditional role of the husband was to be the “breadwinner” outside the home, while the traditional role of the wife was to care for the home and/or the children. Women in general did not have the employment opportunities they have today, and divorced women had an even harder time. Alimony allowed the courts to achieve some measure of fairness by making the husband provide support for his former wife.
In many ways, this view of alimony can seem out-of-place in our modern world. Women have employment opportunities they did not have before, and sometimes the wife actually out-earns the husband. But in many cases, where one spouse has significantly higher earning potential, that has usually been made possible at least in part because the other spouse contributed so much to the marriage. In this way, it can be helpful to see alimony as recoupment of an “investment” in the earning capacity of the other spouse.