Their shows that something belongs to or is connected with a group of people or things, as in 'their house' or 'their ideas'. It is the possessive form of 'they'.
From Old Norse *þeirra* 'of them', which replaced the Old English form *hiera*. Norse and English mixed heavily in the Middle Ages, especially in pronouns like 'they', 'them', and 'their'.
The word 'their' in English is actually a Viking import from Old Norse. When you say 'their', you’re using a tiny fossil of the time when Norse and Old English speakers shared villages and slowly merged languages.
“Their” originated as a plural possessive but has been used as a singular pronoun for centuries. Prescriptive grammarians in the 18th–19th centuries discouraged singular ‘they/their,’ pushing generic ‘he’ instead, which contributed to male-default language norms.
Use “their” as a singular, gender-neutral possessive when a person’s gender is unknown, irrelevant, or nonbinary. This avoids unnecessary gendering and is widely accepted in modern English.
["his or her (formal, now less preferred)"]
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