Entryism is the political strategy which involves the infiltration and subversion of another party with the aim of altering its agenda.

Here in the UK the Trotskyist group Militant was famous for infiltrating the Labour Party and trying to pull it towards the left.

In the editorial of the first issue of the Militant in October 1964, Peter Taaffe made the strategy of entrism clear.

The job is to carry the message of Marxism to the ranks of the labour movement and to its young people. There is room for all tendencies in the labour movement, including the revolutionary Left.

Above all the task is to gather together the most conscious elements in the labour movement to patiently explain the need for these policies on the basis of experience and events.

Militant later became the Socialist Party and the USA's Socialist Alternative is an affiliate to it through the Committee for a Workers' International. I would imagine the discussion surrounding Bernie Sanders is about concerns of Trotskyist infiltration of the Democratic Party in support of Sanders.