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Their
They is the third-person plural personal pronoun (subjective case) in Modern English. It can also be used with singular meaning, particularly in informal contexts, sometimes to avoid specifying the gender of the person referred to.
Wikipedia
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Thier (pronoun)
misspelling of their
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Their (determiner)
Belonging to, from, of, or relating to, them (plural).
“they will meet tomorrow at their convenience;”
“this is probably their cat”
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Their (determiner)
Belonging to someone (one person, singular).
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Their (adverb)
misspelling of there
Wiktionary