Indicates exact position in a long sequence, often used for milestones or numbering editions. We just celebrated our school's millionth day of operation.
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Large Quantities
Used to emphasize the high rank when discussing large quantities, like in production or populations. He was the millionth visitor to the museum.
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Not Literal
Sometimes used hyperbolically to express a large, indefinite number, not always a literal count. I've heard that excuse a millionth time!