Effusive means flowing out of lava as opposed to explosive eruptions.
An effusive vent at Mt Etna volcano
A lava flow is the result of an effusive eruptions (here Mt. Etna in Nov 2006)
If magma is sufficiently fluid and if it is not framented by expanding gasses when reaching the surface vent, it can erupt to form lava flows. This is called effusive eruption. The opposite of effusive is explosive, i.e. fragmentation of magma.
