The philosophical or ethical theory that one's true self exists in another person or that one should value others' interests as much as one's own.
From alter ego (Latin 'other self') plus -ism (ideology/practice suffix). This neologism combines the ancient concept of the alter ego with 19th-century philosophical terminology to describe a systematic ethical philosophy.
This ultra-rare word emerged when philosophers tried to formalize the buddy system—the idea that your best friend represents your 'other self.' It's basically the philosophical foundation of why losing a close friend feels like losing part of yourself!
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