A type of microphone in which the diaphragm is one plate of a capacitor containing an electrical charge. Condenser is an old name for Capacitor.
A thin metal diaphragm is stretched in front of a fixed metal backplate, this forms a capacitor where the capacitance varies as a function of air pressure that moves the front flexible diaphragm.
The charge is either pre-polarised or created with an electrical supply to the microphone. This power is normally supplied as phantom power from the preamplifier for the microphone.