Detchanam literally means south or the land of south. It is also used to represent the Deccan Plateau and south India. In Akilattirattu Ammanai the word Detchanam represents the land of South India with Swamithoppe as its centre, and including a vast area of land, south-east to Kanyakumari.
References
Detchanam Wikipedia(Text) CC BY-SA