Original: The lines between the Bolsheviks and the Mensheviks hardened in April 1905 when the Bolsheviks held a Bolsheviks-only meeting in London, which they called the Third Party Congress.
Simplified: The Party ended up dividing into two groups, the Bolsheviks and the Mensheviks. They were called Bolsheviks because it means "those who are more."