twin talk; cryptophasia; idioglossia
Linguistics term for the private language sometimes developed between twins. The English terms 'twin talk', 'cryptophasia', and 'idioglossia' are all used, with 'cryptophasia' being the most technical.
Twin talk refers to a unique language system understood only between twins.
We studied cases of cryptophasia in linguistics class.
Compound of 双子 (ふたご, 'twins') and 語 (ご, 'language').