The Communist Party of Russia was comprised of two factions of people.
One faction belonged to the moderates who were of the opinion to slowly bring in the social change step by step and ultimately lead the country towards a socialist state. This faction was called the Mansheviks, or “menshinstvo” (minority).
However there was another faction which did not believe in gradual evolution but complete social revolution and these were called the Bolsheviks, or “bolshinstvo” (majority)
Bolshevik, in Russian means: “One of the Majority”: a member of a wing of the Russian Social-Democratic Workers’ Party – the communist party of Russia.
Both these groups: the Bolsheviks and the Mansheviks evolved from the same party.
In the revolution of 1917 it was this Bolshevik faction led by Lenin, that was able to seize control of the government in Russia (October 1917) and became the dominant political power.
The further development of communism in soviet union and elsewhere were carried out by these bolsheviks so the word Bolshevik is often associated with communism.
This approach of complete social revolution is therefore many times referred to as bolshevism.
(compiled from separate sources on quora)