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1917, from Russian bol'shiy "greater," comparative of adjective bol'shoy "big, great" (cf. Bolshoi Ballet), from Old Church Slavonic boljiji "larger," from PIE root *bel- "strong" (cf. Sanskrit balam "strength, force," Greek beltion "better," Phrygian balaios "big, fast," Old Irish odbal "strong," Welsh balch "proud;" Middle Dutch, Low German, Frisian pal "strong, firm").
It was the faction of the Russian Social Democratic Worker's Party after a split in 1903 that was either larger or more extreme (or both) than the Mensheviks (from Russian men'shij "less"); after they seized power in 1917, applied generally to Russian communists. Bolshevism is recorded from 1917.