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While Tacitus called it Mare Suebicum after the Germanic people of the Suebi, the first to name it also as the Baltic Sea ( Mare Balticum ) was eleventh century German chronicler Adam of Bremen.
The origin of the latter name is speculative.
It might be connected to the Germanic word belt, a name used for two of the Danish straits, the Belts, while others claim it to be derived from Latin balteus ( belt ).
However it should be noted that the name of the Belts might be connected to Danish bælte, which also means belt.
Furthermore Adam of Bremen himself compared the Sea with a belt stating that the Sea is named so because it stretches through the land as a belt ( Balticus, eo quod in modum baltei longo tractu per Scithicas regiones tendatur usque in Greciam ).
He might also have been influenced by the name of a legendary island mentioned in The Natural History by Pliny the Elder.
Pliny mentions an island named Baltia ( or Balcia ) with reference to accounts of Pytheas and Xenophon.
It is possible that Pliny refers to an island named Basilia (" kingdom " or " royal ") in On the Ocean by Pytheas.
Baltia also might be derived from " belt " and means " near belt of sea ( strait )".
Meanwhile others have concluded that the name of the island originates from the Indo-European root * bhel meaning white, fair.
This root and its basic meaning were retained in both Lithuanian ( as baltas ) and in Latvian ( as balts ).
On this basis, a related hypothesis holds that the name originated from this Indo-European root via a Baltic language such as Lithuanian.
Yet another explanation is that, while derived from the afore mentioned root, the name of the sea is related to naming for various forms of water and related substances in several European languages, that might have been originally associated with colors found in swamps.
Another explanation is that the name was related to swamp and originally meant " enclosed sea, bay " as opposed to open sea.

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