Note that employers that extend certain benefits to same-sex partners use the word "partner" but generally require cohabitation. Unmarried people living together are sometimes said to be "cohabitating"which suggests "cohabitant"though I've only heard that phase used once and it admits a misunderstanding that it merely means sharing a dwelling.
A possible term for this is significant other.
This is defined as:. The phrase live-in girlfriend or boyfriend is sometimes used. Domestic partner also describes the situation, but in recent years it has come to refer to homosexual partnerships more often than heterosexual ones.
The United States Census Bureau originated a phrase which became quite popular circa or so: Such a bureaucratic, unromantic phrase Basically, if you act like a married couple, and you're just missing the official piece of paper, you can get away with using words like husbandwifeand others that are typically reserved for legal marriage. To inject a personal anecdote, after my father died, my mother began seeing, and eventually living with, another man.
They had intended to get married, but things got in the way planning my sister's marriage, then illnesses and they eventually decided it wasn't really necessary. But after a while, she started referring to him as her second husband, and everyone accepted it. She'd become close to his family, considered his children all grown to be hers.
When his grandson had a child, she announced that she was a great-grandmother. Here in Australia at least, this would be referred to as a de facto partnerde facto being a Latin expression meaning 'by fact'. That is, a partnership for all intents and purposes but not a marriage by law.