The Balts or Baltic people is an Indo-European ethnic-linguistic group that speaks Baltic languages, a branch of the Indo-European language family, which was originally spoken by tribes living in the area of Jutland peninsula in the west and Moscow, Oka and Volga rivers basins in the east. One of the features of Baltic languages is the number of conservative or archaic features retained. Among the Baltic peoples are modern Lithuanians, Latvians (including Latgalians) — all Eastern Balts — as well as the Prussians, Yotvingians and Galindians — the Western Balts — whose languages and cultures are now extinct.
Adam of Bremen was the first writer to use the term Baltic in its modern sense to mean the sea of that name. Although he must have been familiar with the ancient name, Balcia, meaning a supposed island in the Baltic Sea, and although he may have been aware of the Baltic words containing the stem balt-, "white", as "swamp", he reports that he followed the local use of balticus from baelt ("belt") because the sea stretches to the east "in modum baltei" ("in the manner of a belt"). This is the first reference to "the Baltic or Barbarian Sea, a day's journey from Hamburg."
The Germanics, however, preferred some form of "East Sea" (in different languages) until after about 1600, when they began to use forms of "Baltic Sea." Around 1840 the German nobles of the Governorate of Livonia devised the term "Balts" to mean themselves, the German upper classes of Livonia, excluding the Latvian and Estonian lower classes. They spoke an exclusive dialect, baltisch-deutsch, legally spoken by them alone. For all practical purposes that was the Baltic language until 1919. Scandinavians begin settling in Western Baltic lands in Lithuania and Latvia.
Meanwhile in 1845 Georg Heinrich Ferdinand Nesselmann proposed a distinct language group for Latvian and Lithuanian to be called Baltic. It found some credence among linguists but was not generally adopted until the creation of the Baltic states as part of the settlement of World War I in 1919. Gradually the non-Baltic Estonian was excluded from the linguistic meaning of Baltic, as was Livonian, a now rare Finnic language in Latvia, while Old Prussian — long recognized as close to Lithuanian and Latvian — was added. Estonia remained, however, among the Baltic states in the geopolitical sense.
Read more about Balts: Proto-history, History, Summary of Baltic Peoples and Tribes