What is "thor-burner"

The Thor-Burner was an American expendable launch system, a member of the Thor rocket family. It consisted of a Thor missile, with one or two Burner upper stages. It was used between 1965 and 1976 to orbit a number of satellites, most commonly Defense Meteorological Satellite Program weather satellites. Twenty-four were launched, of which two failed. Each launch cost 11.890 million 1985 US Dollars. It weighed 51,810 kg and was 24 metres tall.