According to mead's theory of development, individuals who have a profound influence on the lives of another person, such as parents or siblings, are referred to as

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They are referred to as "the significant others".

In common term, significant other might mean the romantic partner. However in sociology, significant others has a deeper meaning. These include those persons who are important enough to affect the person’s emotions, behavior, and sense of one self. This could be the family, friends, or mentors.