esoteric language
Definition - A language that is used primarily for within-group communication and is spoken by few if any outsiders (e.g. Navaho), hence it tends to isolate the in-group from surrounding groups.

As opposed to exoteric languages (e.g. English, French, and Mandarin), which are lingua francas used for between-group communication.

1. Esoteric languages tend to become more complex over time, thus become increasingly difficult for outsiders to learn. In contrast, exoteric languages tend to become less complex over time as the complexities are worn down by outsiders trying to learn them.

Etymology -
The term was introduced by the linguist William R. Thurston.

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