Why was a capacitor called a condensor (condenser?) in the early days of electronics?

As the term has been traced (thanks to @helloworld922) back to 1782, it's worth noting this is the year James Watt patented the compound steam engine, having conceived the separate condenser in 1765, and patented it and produced efficient condensing steam engines in the 1770s.

So the term was very much cutting edge at the time, and scientists tended to read much more widely across disciplines than we can possibly do today, so certainly Volta would have been aware of it.

In those days, electrical concepts were explained by analogy with fluid flow concepts, with pressure corresponding to voltage and current corresponding to ... current.

So, because a condenser absorbs large volumes of steam at very low pressure, it offers a good analogy for a device which can absorb a lot of charge at relatively low electrical pressure. (However the analogy breaks down when you try to recover the steam : the condenser can only deliver water!)

Interesting, while books of a hundred years ago talk of electrical pressure (measured in volts) and electrical current (measured in amps) we have dropped the former term in favour of "voltage", it still looks odd to see "amperage" instead of the word "current", and I can't recall seeing "ohmage" in place of "resistance".

The "Admiralty Handbook of Wireless Telegraphy" (1925 edition) consistently uses the term "condenser" while calling its storage capacity "capacity". The book introduces both the "practical unit" of the Farad, (millifarad, microfarad, and micromicrofarad, so apparently "pico" wasn't in use yet) and the "service unit" of the Jar. (by 1925, "electrical pressure" has given may to"Electro-Motive Force" or EMF, which is still occasionally seen in the wild today)

The original condensers were actually glass jars (Leyden jars), presumably of a standard size, because the book introduces the "service unit" which is the Jar, where 1 Jar = 1/900 uF. (It then goes on to inconsistently use jars and farads throughout the remainder of the book!)

So we have consistently dropped some of the contemporary terms, kept some others, and inconsistently dropped others - "condenser" is still the term in the spare parts catalog for my outboard motor while "capacitor" is seen elsewhere.

It seems the word comes from the latin condenseo which means to condense or to compress.

This does make sense because in contrast to a piece of wire, you can push charge into the cap without too much pressure (voltage). It seems the charge condenses inside like propane gas does, when it is pressed into a gas bottle.

By the way, the german word is Kondensator, and it has a Kapazität.

The term condenser is still used for the capacitor in older automotive ignition systems. These used the condenser along with a 'coil' (as step-up transformer) and points (mechanical switch), to generate the spark from the 6 or 12 VDC available in engine charging systems. In modern cars the spark is generated electronically. AFAIK the mechanical systems were on their way out in the 80's, completely gone by the 90s as computer electronics took over most engine functions. But you can still buy condensers+points for old cars.