The difference between One-on-one and One-to-one
When used as nouns, one-on-one means a contest involving only one player on each side, especially of an activity often involving teams, whereas one-to-one means a personal relationship between two people.
When used as adjectives, one-on-one means involving direct communication between two people, whereas one-to-one means matching each member of one set with exactly one member of another set.
check bellow for the other definitions of One-on-one and One-to-one
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One-on-one as an adjective (chiefly, North America):
Involving direct communication between two people.
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One-on-one as an adjective (sports):
involving one attacker and one defender
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One-on-one as an adjective (mathematics):
bijective or injective
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One-on-one as a noun:
A contest involving only one player on each side, especially of an activity often involving teams.
Examples:
"Why don't we play a little one-on-one until the others show up."
"He was willing to go one-on-one with the District Attorney himself."
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One-to-one as an adjective:
Matching each member of one set with exactly one member of another set.
Examples:
"There is a one-to-one relationship between days with large cash shortages and his workdays."
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One-to-one as an adjective (chiefly, UK):
Involving direct communication between two people.
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One-to-one as an adjective (mathematics, of a function):
Injective, being an injection: having the property that no two elements of the domain are mapped to the same image.
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One-to-one as a noun:
A personal relationship between two people.