Not married — with no husband or wife. It sounds neutral and factual, which is exactly why it is the word used on forms, in news reports, and in official writing: 'an unmarried mother', 'he died unmarried at the age of sixty'. Compare it with 'single', which is the ordinary spoken word and usually means the person has no partner at all; 'unmarried' says only that no wedding has happened, so a couple who have lived together for thirty years are still, technically, unmarried. That gap is worth knowing, because writers use it. Note also that the word points backwards in a way 'not married yet' does not: because it is built on 'married', it quietly measures a person against marriage as the expected state. That is why older phrases like 'an unmarried woman of forty' feel disapproving — the disapproval is in the assumption, not in the letters. Modern writing has largely moved to 'single' or simply says nothing. There is a rarer noun use, 'the unmarried', meaning unmarried people as a group, which you meet mostly in law, statistics, and church writing.