Lake Geneva is a deep lake on the north side of the Alps, shared between Switzerland and France. It is one of the largest lakes in Western Europe and the largest on the course of the Rhône. Sixty percent (345.31 km² or 133.32 sq mi) of the lake belongs to Switzerland (the cantons of Vaud, Geneva and Valais) and forty percent (234.71 km² or 90.62 sq mi) to France (the department of Haute-Savoie).

While the exact origins of the name are unknown, the name Lacus Lemanus was in use during the time of Julius Caesar. Lemannus comes from Ancient Greek Liménos Límnē (Λιμένος Λίμνη) meaning "port's lake".^{[citation needed]} In Medieval Latin it was known as Lacus Lausonius, although this name was also used for a town or district on the lake, or Lacus Losanetes; the equivalent in Old French was Lac de Lausanne.^{[citation needed]} Following the rise of Geneva it became Lac de Genève (translated into English as Lake Geneva), but Le Léman was the common name on all local maps and is the customary name in the French language. In contemporary English, the name Lake Geneva has become predominant.

Lake Geneva is divided into three parts because of its different types of formation (tectonic folding, glacial erosion, sedimentation):

According to the Swiss Federal Office of Topography, Swisstopo, Lac de Genève designates that part of the Petit Lac which lies within the cantonal borders of Geneva (excluding the cantonal exclave Céligny), so about from Versoix–Hermance to the Rhône outflow in Geneva.

The Chablais Alps border is its southern shore, and the western Bernese Alps lie over its eastern side. The high summits of Grand Combin and Mont Blanc are visible from some places.

The lake lies on the course of the Rhône. The river has its source at the Rhône Glacier near the Grimsel Pass to the east of the lake and flows down through the canton of Valais, entering the lake between Villeneuve and Le Bouveret, before flowing slowly towards its egress at Geneva. Other tributaries are La Dranse, L'Aubonne, La Morges, La Venoge, La Vuachère, and La Veveyse.

Lake Geneva is the largest body of water in Switzerland, and greatly exceeds in size all others that are connected with the main valleys of the Alps. It is in the shape of a crescent, with the horns pointing south, the northern shore being 95 km (59 mi) and the southern shore 72 km (45 mi) in length. The crescent form was more regular in a recent geological period, when the lake extended to Bex, about 18 km (11 mi) south of Villeneuve. The detritus of the Rhône has filled up this portion of the bed of the lake, and it appears that within the historical period, the waters extended about 2 km (1.2 mi) beyond the present eastern margin of the lake. The greatest depth of the lake, in the broad portion between Évian-les-Bains and Lausanne, where it is just 13 km (8.1 mi) in width, has been measured as 310 m (1,020 ft), putting the bottom of the lake at 62 m (203 ft) above sea level. The lake's surface is the lowest point of the cantons of Valais and Vaud. The culminating point of the lake's drainage basin is Monte Rosa at 4,634 metres above sea level.

The beauty of the shores of the lake and of the sites of many of the places near its banks has long been celebrated. However, it is only from the eastern end of the lake, between Vevey and Villeneuve, that the scenery assumes an Alpine character. On the south side, the mountains of Savoy and Valais are for the most part rugged and sombre, while those of the northern shore fall in gentle vine-covered slopes, thickly set with villages and castles.