Superheated liquid is a liquid which is not undergoing phase transition into gas in bulk (=boiling) even if its temperature is *higher* (that's why it is called *super*heated) than the temperature sufficient for this phase transition to occur (high enough temperature means high enough vapor pressure which can overcome external pressure). Superheated liquid  is metastable state, a strong enough mechanical excitation or introduction of nucleation facilitators (dirt, radiation, charged particles) can destroy it and the phase transition then will occur.