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significant other vs. girlfriend

noun

  1. Sociology.  a person, as a parent or peer, who has great influence on one's behavior and self-esteem.

  2. a spouse or cohabiting lover. SO, S.O.

[gurl-frend] / ˈgɜrlˌfrɛnd /
Or girl friend

noun

  1. a frequent or favorite female companion; sweetheart.

  2. a female friend.