Cryptophasia

Cryptophasia is the phenomenon of a language developed by twins (identical or fraternal) that only the two children can understand. The word has its roots from the Greek crypto-, meaning secret, and -phasia, meaning speech. Most linguists associate cryptophasia with idioglossia, which is any language used by only one, or very few, people. Cryptophasia differs from idioglossia on including mirrored actions like twin-walk and identical mannerisms.