A court status in which spouses live apart with binding orders on support, property and children, while remaining married.
A legal separation is a court proceeding that resolves substantially the same questions a divorce does - support, property, parenting - but ends with the parties still married. It is not the same as simply living apart, which changes nothing about anyone's legal position, and it is not available in every state.
People choose it for reasons that are usually practical rather than sentimental: continuing eligibility for a spouse's health coverage, religious objections to divorce, or immigration and pension consequences that turn on marital status. The trade-off is that neither person may remarry, and in many states the financial orders made in a separation carry over into a later divorce.
Separately from all of this, a handful of states recognise informal or common-law marriage, and a few treat a period of living apart as relevant to a later divorce. Those are state-specific rules, and assuming a national answer to either question is a common and expensive mistake.
The reason to take advice before separating rather than after is that conduct during the separation - who moves out, who pays what, whose name is on which account - frequently affects the eventual outcome, and by the time a divorce is filed those facts are already established. Anyone separating primarily to keep a health or pension benefit should also have someone confirm that the benefit actually survives separation, because plan terms vary and the assumption is not always right.
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