The Zuiderzee or Zuider Zee (Dutch: [ˌzœydərˈzeː] ^ⓘ; old spelling Zuyderzee or Zuyder Zee), historically called Lake Almere and Lake Flevo, was a shallow bay of the North Sea in the northwest of the Netherlands. It extended about 100 km (62 mi) inland and at most 50 km (31 mi) wide, with an overall depth of about 4 to 5 m (13 to 16 ft) and a coastline of about 300 km (190 mi). It covered 5,000 km² (1,900 sq mi). Its name is Dutch for "southern sea", indicating that the name originates in Friesland, to the north of the Zuiderzee (cf. North Sea).^{[citation needed]}

It is generally acknowledged that the Zuiderzee existed from roughly 1170, following the devastating All Saints' Flood, until 1932, when the Afsluitdijk was completed. The majority of the Zuiderzee was closed off from the North Sea, leaving the mouth of the inlet to become part of the Wadden Sea. The salt water inlet changed into a fresh water lake now called the IJsselmeer (IJssel Lake) after the river that drains into it, and by means of drainage and polders, an area of some 1,500 km² (580 sq mi) was reclaimed as land. This land eventually became the province of Flevoland. Part of the IJsselmeer was also divided into the Markermeer.

In classical times the body of water at this location was called Lake Flevo (Flevo Lacus) by Roman authors. This was the central and largest lake in a region filled with a mixture of lowland and freshwater lakes occupying the area later filled by the Zuiderzee. It was separated from the sea by a belt of marsh and fen; at that time, the coastline ran along the line of the Frisian Islands. A number of streams, including the Vecht, Eem, and Ysel, fed into the lake. The lake itself fed out into the North Sea through the Vlie (Latin: Flevus). It existed in Roman times and the early Middle Ages.

From the Indo-European root *plew- "flow", the name was transmitted by the Roman geographer Pomponius Mela in describing this region. In his treatise on geography of 44 AD, Pomponius speaks of a Flevo Lacus. He writes: "The northern branch of the Rhine widens as Lake Flevo, and encloses an island of the same name, and then as a normal river flows to the sea". Other sources rather speak of Flevum, which could be related to today's Vlie (Vliestroom), i.e. the seaway between the Dutch islands of Vlieland and Terschelling. This last name is grammatically more probable for a geographical indication, which is why it is assumed that Pomponius confused the declension of the word giving the name Flevo. In fact the Vlie formed outfall from the lake into the North Sea.^{[citation needed]}

In the second half of the twentieth century the Flevopolders and a new province, Flevoland, took the name of the body of water which lay there long ago.^{[citation needed]}

Over time these lakes gradually eroded their soft peat shores and spread (a process known as waterwolf). Some part of this area of water was later called the Vlie; it probably flowed into the sea through what is now the Vliestroom channel between the islands of Vlieland and Terschelling.

During a period of rising sea level between 250 and 600 CE, the connection to the sea and the lake were significantly enlarged. A period of lower sea levels followed.

Lake Almere is mentioned among others in a life of saints written by Anglo-Saxon Bishop Saint Boniface in 753, and a deed of gift from the town of Urk.^{[citation needed]} Its etymology may be eels, in Dutch aal or ael, so: ael mere = "eel lake".^{[citation needed]} Presumably, the water of Lake Almere at that time was fresh water or slightly brackish.^{[citation needed]} The name of the new town of Almere in Flevoland was given in 1984 in memory of this body of water.^{[citation needed]}