nit-picking aside the definition still holds true, that is the reactor is operating on close or above peak output with the cooling inoperable. Once that situation occurs and you can't do anything about it you'll have a meltdown.
The situation is so completely different than what happens in a traditional reactor that it's deceptive to use the same word to describe it.
First of all the reactor couldn't operate at full power with no cooling for more than a very short period of time because of physics. High temperatures shut down the fission reaction.
Even if there was fracture in the reactor vessle or associated piping you wouldn't get an explosion, fire, uncontrolled criticiality resulting in a "China Syndrome", or massive release of airborne radioactivitiy
There would be a mess on the floor which would cool and freeze into an inert solid.