Which countries territory is Christmas Island?

There is more than one Christmas Island.

The Territory of Christmas Island is a small territory of Australia located in the Indian ocean, 2,360 kilometres (1500 mi) northwest of Perth in Western Australia and 500 kilometres (300 mi) south of Jakarta, Indonesia. This island has been administered as an external territory of Australia since 1958. It is arguably the most well known, being used as a detention centre for asylum seekers.

There is also Christmas Island, Nova Scotia, Canada.

Also there is Kiritimati, which is an atoll also known as Christmas Island, in the Pacific Ocean. It is part of the Republic of Kiritimati.

Finally there is Christmas Island, just west of Australia's island state of Tasmania. This island lies in the Southern Ocean.