The full form of TURN is Traversal Using Relays around NAT

Traversal Using Relays around NAT (TURN) is a protocol that assists in traversal of network address translators (NAT) or firewalls for multimedia applications. It may be used with the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). It is most useful for clients on networks masqueraded by symmetric NAT devices. TURN does not aid in running servers on well known ports in the private network through a NAT; it supports the connection of a user behind a NAT to only a single peer, as in telephony, for example.

TURN is specified by RFC 5766. An update to TURN for IPv6 is specified in RFC 6156. The TURN URI scheme is documented in RFC 7065