Alimony is called maintenance or spousal maintenance in England & Wales. It's one of the divorce financial orders a court can make. It is mainly designed to bridge the gap between the reasonable expenditure needs of a spouse and their income where the income of the other spouse is higher. The overall aim on a divorce is to have a clean break order wherever possible. If that can't be done immediately, alimony is often payable only for the limited period of time that is needed to enable the receiving spouse to readjust. This is called a term order. In some cases you can't predict if or when financial independence is going to be achieved in which case the maintenance can be open-ended. This is called a joint lives order.
Alimony is always capable of being varied to take account of changing circumstances. It keeps a couple linked together financially which is something the courts and most couples will want to try to avoid, which is why a clean break order is the starting point.