Formed to unite the various revolutionary organizations of the Russian Empire into one party in 1898, the RSDLP later split into Bolsheviks (majority) and Mensheviks (minority) factions, with the Bolshevik faction eventually becoming the Communist Party of the Soviet Union. The Interdistrictites, known as the Russian Social Democratic Labor Party (Internationalists), were also formed from this party.
The RSDLP was not the first Russian Marxist group as the Emancipation of Labour group was formed in 1883. The RSDLP was created to oppose the Narodniks revolutionary populism, which was later represented by the Socialist Revolutionary Party (SRs). The RSDLP program was based on the theories of Karl Marx and Friedrich Engels, namely that despite Russia's agrarian nature, the true revolutionary potential lay with the industrial working class. The RSDLP was illegal for most of its existence as at the end of the 1st Party Congress in March 1898 all nine delegates were arrested by the Imperial Russian Police. At this time, there were 3 million Russian industrial workers, just 3% of the population.
Before the 2nd Party Congress, a young intellectual named Vladimir Ilyich Ulyanov joined the party, better known by his pseudonym—Vladimir Lenin. In 1902, he had published What Is To Be Done?, outlining his view of the party's task and methodology—to form "the vanguard of the proletariat". He advocated a disciplined, centralized party of committed activists who sought to fuse the underground struggle for political freedom with the class struggle of the proletariat.
In 1903, the 2nd Party Congress met in exile in Brussels to attempt to create a united force. However, after unprecedented attention from the Belgian authorities the Congress moved to London, meeting on 11 August in a chapel in Tottenham Court Road. At the Congress, the party split into two irreconcilable factions on 17 November: the Bolsheviks (derived from bolshinstvo—Russian for "majority"), headed by Lenin; and the Mensheviks (from menshinstvo—Russian for "minority"), headed by Julius Martov. Confusingly, the Mensheviks were actually the larger faction, but the names Menshevik and Bolshevik were taken from a vote held at the 1903 Party Congress for the editorial board of the party newspaper, Iskra (Spark), with the Bolsheviks being the majority and the Mensheviks being the minority. These were the names used by the factions for the rest of the party Congress and these are the names retained after the split at the 1903 Congress. Lenin's faction later ended up in the minority and remained smaller than the Mensheviks until the Russian Revolution.
A central issue at the Congress was the question of the definition of party membership. Martov proposed the following formulation: "A member of the Russian Social-Democratic Labour Party is one who accepts the Party’s programme, supports the Party financially, and renders it regular personal assistance under the direction of one of its organizations". On the other hand, Lenin proposed a more strict definition: "A member of the Russian Social-Democratic Labour Party is one who accepts its programme and who supports the Party both financially and by personal participation in one of the Party organizations". Martov won the vote and the Bolsheviks accepted it as part of the adopted organizational rules.