Occurring or existing at exactly the same time; simultaneous in the most precise sense.
From co- (together) + instantaneous (from Latin instantia). A learned term from philosophical discourse, roughly 17th century, emphasizing perfect temporal overlap rather than mere simultaneity.
While 'simultaneous' means roughly at the same time, 'coinstantaneous' means at literally the exact same instant—no millisecond difference. It's the kind of term philosophers used when they really wanted to be precise about time.
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