/ ðeə(r); NAmE ðer/ determiner (the possessive form of they )
1.
of or belonging to them :
Their parties are always fun.
Which is their house?
2.
used instead of his or her to refer to a person whose sex is not mentioned or not known :
If anyone calls, ask for their number so I can call them back.
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WORD ORIGIN
Middle English : from Old Norse their(r)a of them, genitive plural of the demonstrative sá ; related to them and they .