What is a passive preamp?

Think of it as an input switcher with a volume control :-)

there is no active gain in a passive preamp. it's just an attenuator with switching capability. so you can select the input source and turn the volume down but not up. since there is no active circuits, many believe the sound to be of utmost transparency but there may be impedance mis-matching and lack of dynamics if cable runs are long as the signal strength will attenuate down the cable path.

A passive pre-amp doesn't amplify the signal. It only attenuates the signal from the source.

A "Passive pre-amp" can come with one or more inputs but the use of the name "passive pre-amp" is inappropriate. Your impression is correct. It is a passive 'line stage', or in the case of a single input its simply an inline volume control/attenuator.

But there are preamps with no gain that are not passive.

There can be active buffering without gain, so the "no-gain" thing is not what defines what's passive or not.

It's not really a preamp without a phono stage. And it can't really be called a preamp if it doesn't pre-amplify. It would really be a passive linestage.