Ask Lemmygrad

Academic Marxism is the tendency to study Marxism solely as an economic theory without little to no organizative theory or practice, rendering it toothless.

This means that the beginning and end of the organisation's work is confined to universities, particularly economics and social science departments, bringing with that all the petty bourgeois and elitist trends in academia. In short, it's "people who only read Marx in German, but never went to a picket line".

This is extremely common in bourgeois democracies as a way of institutionalising critique, and therefore making it harmless. Rather than making communism illegal, the ruling class makes effective party work illegal, but "tolerates" intellectual Marxists with high pay, healthcare and good benefits.

For further reading, here's an (academic) article critiquing academic Marxists and warning of how actual militant workers movements are in danger of being co-opted by liberal ideology in 1977. Ronald Reagan was elected in 1978.