The ordinal number that comes after 999,999; something that is number one million in a sequence.
From 'million' (from Old French 'million,' originally meaning 'a thousand thousand') plus the ordinal suffix '-th.' The suffix '-th' comes from Old English and is used to turn cardinal numbers into ordinal numbers (one→first, two→second, one million→millionth).
Our brains can't really imagine what a millionth of anything feels like—it's so small it's almost impossible to visualize. Yet we have the word for it, showing how language lets us describe things that transcend human intuition or experience.
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