significant other
US /sɪɡˈnɪf.ɪ.kənt ˈʌð.ər/
UK /sɪɡˈnɪf.ɪ.kənt ˈʌð.ər/

1.
partenaire, moitié
a person with whom someone has a long-term, committed relationship, especially one who is not a spouse but is considered a partner in life
:
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She introduced him to her significant other at the party.
Elle l'a présenté à son partenaire à la fête.
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Many people prefer to use 'significant other' instead of 'boyfriend' or 'girlfriend' to be more inclusive.
Beaucoup de gens préfèrent utiliser 'partenaire' au lieu de 'petit ami' ou 'petite amie' pour être plus inclusifs.