theirs S3 [possessive form of 'they']
used to refer to something that belongs to or is connected with people that have already been mentioned:
When our washing machine broke, our neighbours let us use theirs.
Our house is number 25, and theirs is just opposite.
We compared scores and found that theirs were higher than ours.
used when talking about someone who may be male or female, to avoid saying 'his or hers':
Everyone wants what is theirs by right.