jonbar hinge
noun/ˈd͡ʒɒnbɑɹ ˈhɪnd͡ʒ/
Etymology A reference to the novel The Legion of Time (1938), in which the character John Barr having the choice of picking up of one of two objects (a magnet or a pebble) marks a major turning point in history: one will lead to the utopian empire of Jonbar (named after John Barr himself), while the other leads to the rise of the tyrannical state of Gyronchi. Further etymology at John, Barr.
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Literature, Media, Publishing, Science fiction In time travel stories: a seemingly trivial event which ultimately marks the point of divergence between two radically different timelines.