The Tholian sector was a region of space. In the late 2260s, this sector was unsurveyed. An area within the sector was, according to the Federation, free space. This contrasted strongly with the opinion of the Tholian Assembly, the governing authority of the Tholians, who considered this area as a territorial annex. In 2268, the starship USS Defiant was sent to this sector on a survey mission. During the course of its survey, the ship became trapped in a spatial interphase.