significant other

noun

1. One's exclusive romantic partner.

   “Almost half of working women who are married or live with a partner are seeing their significant other only in passing because the two work different shifts, an AFL-CIO poll found.”

   “More than half (57 percent) of consumers think it would be wonderfully romantic if their significant other booked tickets for a surprise international trip.”

2. A person with whom one has an important bond of some kind.

   “Dr. Nogueira said it was always preferable to have coaches drawn from a school's staff because a coach "is a significant other" to a student.”

   “As in the rest of the hospital, a family member or "significant other" such as a lover or close friend, is allowed to spend the night in a patient's room on a cot.”