Cryptophasia is a peculiar phenomenon of a language developed by twins (identical or fraternal) that only the two children could understand. The word has its roots from crypto meaning secret and phasia meaning speech disorder. Most linguists associate cryptophasia with idioglossia

Idioglossia

An idioglossia is an idiosyncratic language invented and spoken by only one person or very few people. Most often, idioglossia refers to the "private languages" of young children, especially twins, the latter being more specifically known as cryptophasia, and commonly referred to as twin talk or...

, which is literally the same, but cryptophasia also includes mirrored actions like twin-walk and identical mannerisms. Little is known about cryptophasia.

While sources claim that twins and children from multiple births develop this ability perhaps because of more interpersonal communication between themselves than with the parents, there is inadequate scientific proof to verify these claims.