Source: Social Democracy: What it is & how Communists relate to it

“When Lenin came back from exile to Russia that was in its revolutionary crisis, he urged the Bolsheviks to stop calling themselves Socialists and Social Democrats because he didn’t want them to be confused with the sellout parties of the Second International. The parties that had supported World War I and sold out the revolution. So the Bolsheviks started calling themselves Communists to distinguish themselves from the parties of the Second International.” — Caleb Maupin