Baltic.

From Latin Balticus, from Latin Balthae (“dwellers near the Baltic sea”); equivalent to Balt + -ic. The ultimate origin of the ethnonym is unknown, but possibilities include: * Related to Lithuanian baltas (“white”) and Proto-Slavic *bolto (“swamp, bog, mud”), which are likely from Proto-Indo-European *bʰel(H)- (“white”) via Proto-Balto-Slavic *bálˀtas (“white”). This supposes a semantic shift in Baltic from “white” to something like “marsh” or “sea”, paralleling that in the Slavic cognate. (However, the only such parallels attested from Baltic occur in Lithuanian balà (“swamp, mire, morass; puddle”) and Latvian bala (“loamy valley without trees, swamp, depression”), which are not only semantically more distant but crucially lack the *-t- suffix.) On the other hand, similar words for “swamp, marsh” throughout Europe have been considered substrate words; more at *bolto and baltë. * From a hypothetical derivative of Proto-Germanic *baltijaz (itself borrowed from Latin balteus (“belt”)), originally referring to the Danish straits, "the Belts". This is suggested by Adam of Bremen, who in the 11th century first recorded the name (Balticus, eo quod in modum baltei longo tractu per Scithicas regiones tendatur usque in Greciam). All of these options involve a great deal of speculation, however. More on Wikipedia.

From Latin Balticus, from Latin Balthae (“dwellers near the Baltic sea”); equivalent to Balt + -ic. The ultimate origin of the ethnonym is unknown, but possibilities include: * Related to Lithuanian baltas (“white”) and Proto-Slavic *bolto (“swamp, bog, mud”), which are likely from Proto-Indo-European *bʰel(H)- (“white”) via Proto-Balto-Slavic *bálˀtas (“white”). This supposes a semantic shift in Baltic from “white” to something like “marsh” or “sea”, paralleling that in the Slavic cognate. (However, the only such parallels attested from Baltic occur in Lithuanian balà (“swamp, mire, morass; puddle”) and Latvian bala (“loamy valley without trees, swamp, depression”), which are not only semantically more distant but crucially lack the *-t- suffix.) On the other hand, similar words for “swamp, marsh” throughout Europe have been considered substrate words; more at *bolto and baltë. * From a hypothetical derivative of Proto-Germanic *baltijaz (itself borrowed from Latin balteus (“belt”)), originally referring to the Danish straits, "the Belts". This is suggested by Adam of Bremen, who in the 11th century first recorded the name (Balticus, eo quod in modum baltei longo tractu per Scithicas regiones tendatur usque in Greciam). All of these options involve a great deal of speculation, however. More on Wikipedia.