The meridian 100° west of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, North America, the Pacific Ocean, the Southern Ocean, and Antarctica to the South Pole.

The 100th meridian west forms a great circle with the 80th meridian east.

Starting at the North Pole and heading south to the South Pole, the 100th meridian west passes through:

In the United States the meridian 100° west of Greenwich forms the eastern border of the Texas panhandle with Oklahoma (which traces its origin to the Adams-Onís Treaty in 1819 which settled the border between New Spain and the United States between the Red River and Arkansas River). Dodge City, Kansas lies exactly at the intersection of the Arkansas River and the 100th meridian.

In the central Great Plains, the meridian roughly marks the western boundary of the normal reach of moist air from the Gulf of Mexico, and the approximate boundary (although some areas do push the boundary slightly farther east) between the semi-arid climate to the west and the humid continental (north of about 37°N) and humid subtropical (south of about 37°N) climates to the east. The type of agriculture west of the meridian typically relies heavily on irrigation. Historically the meridian has often been taken as a rough boundary between the eastern and western United States. White settlement, spreading westward after the American Civil War, encroached in the meridian in the 1870s.

ITV Meridian, previously Meridian Broadcasting from its inception in 1993 until 2002, is the holder of the ITV franchise for the South and South East of England. The station launched on 1 January 1993, replacing previous broadcaster Television South, and is presently owned and operated by ITV plc, under the licensee of ITV Broadcasting Limited.

The company broadcasts to the region from the Hannington, Midhurst, Rowridge, Whitehawk Hill, Hastings, Heathfield, Tunbridge Wells, Bluebell Hill and Dover transmitters. The company also broadcasts their news service to viewers served by the Oxford transmitting station despite this area being in the neighboring ITV Central region.

Meridian operates from Whiteley, Hampshire, producing regional news and current affairs programmes in addition to occasional networked productions for the ITV network.

ITV Meridian was formed as Meridian Broadcasting in 1991. The company consisted of a consortium between Mills & Allen International (MAI), Selectv (15%) and Central Independent Television (20%), the latter advised the consortium. Central along with Selectv were instrumental in winning the franchise as a low bidder. Meridian intended to operate as a publisher broadcaster, meaning that the majority of programmes would be commissioned from independent producers and not made by the company.

100th meridian may refer to: