significant other

noun2

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. — 2000
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. — 1983