Cryptophasia is a phenomenon of a language developed by twins (identical or fraternal) that only the two children can understand. The word has its roots from cryptomeaning secret and phasia meaning speech. Most linguists associate cryptophasia with idioglossia, which is any language used by only one, or very few, people. Cryptophasia also differs from idioglossia on including mirrored actions like twin-walk and identical mannerisms.
It has been reported that up to 50% of young twins will have their own twin language which they use to communicate only with each other and one that can not be understood by others