“Resistor” usually implies a device with a fixed resistance value. A rheostat is a device with variable resistance. The two terms are not synonymous.

As for condenser and capacitor, Wikipedia has an interesting tidbit:

Early capacitors were known as condensers, a term that is still occasionally used today, particularly in high power applications, such as automotive systems. The term was first used for this purpose by Alessandro Volta in 1782, with reference to the device’s ability to store a higher density of electric charge than was possible with an isolated conductor. The term became deprecated because of the ambiguous meaning of steam condenser, with capacitor becoming the recommended term in the UK from 1926, while the change occurred considerably later in the United States.

https://en.wikipedia.org/wiki/Capacitor

This seems like a fun rabbit hole to go down with regard to capacitors vs condensers, but a rheostat and a resistor are not the same, and both are used by these names in electronics today.

Another player is the potentiometer, which handles low power variability. That’s about the extent of my ability to shed light here though. I look forward to other more knowledgeable than I am adding to (and likely correcting) my comments.

Edit: typo and clarification

Edit2: A family member was re-assembling an audio amp and set a 1-farad cap on his just wiped countertop (which happened to be damp still) and blew a crater in the cheap Formica. We laughed, and nobody was hurt. I was 14, and learned that a capacitor sure can discharge quickly!

Rheostats are a type of variable resistor, commonly seen in the form of potentiometers.

“Condenser” came from the analogy of steam power’s condenser, which was a recent/new thing for steam engines around the same time people were starting with electricity. Language changed, though, because the analogy was imperfect.

Increased specificity. As someone else mentioned rheostats are variable, which is implied by the suffix -stat.

But also electricity is already hard to learn when everything is named after what it does. If you’re working with circuits a lot you’re gonna start calling resistors resistors because they provide resistance and engineers are like that. Similarly capacitors entire thing is revolving around a set capacity for charge that we call capacitance. I know impedance devices have their own special name but by the gods I really want to call them impeders or “that coil thing with the electromagnetic slowing”. I’m not an EE, I rarely fuck with electricity but yeah eventually they were gonna get called these things as we got to understand them better as the fundamental building blocks of circuitry.

Also condensers are a different thing in thermo so that may contribute here.

To add to the other answers you’ve gotten, “cycles” and “hertz” are both still used. The frequency (in Hz) is a count of how many cycles are in a one second period. A datasheet for an electronic device might have the frequency it’s compatible with listed on it (typically 60Hz in the US, 50Hz in Europe).

For some signal processing and protection equipment you’d also see a number of cycles listed on the datasheet - that will always be paired with a “at X Hz” clarification, because it’s functionally telling you how long the device takes to operate. For utility line circuit breakers, for example, “3 cycle” and “5 cycle” breakers are the most common options in the US, where 3 cycles translates to “this breaker will be fully open within 3/60 of one second of a trip command.”

I’m going to take a first stab at some parts of the question.

Regarding rheostat vs resistor, I don’t think one term replaces the other. In modern terminology, a rheostat refers to a two-lead device that varies in resistance. Whereas a resistor implies a fixed resistance. Rheostat brakes would make sense, since a fixed amount of braking current would be… unusual. “Variable resistance brakes” would mean the same, but is longer.

For cycles vs Hertz, I’ve not personally come across a technical reference which only listed “cycles”. Rather, old radios often list “cycles per second” when documenting the intermediate frequency, for example. So compared to writing “cycles per second” or “cps” over and over, Hertz is much shorter and easily abbreviates as Hz (eg MHz, kHz).

For condenser vs capacitor, I honestly haven’t any idea. I’m also keen to see some other answers to this question.

It’s most likely as our understanding of the systems and the underlying physics have changed, so have the terminologies.
There was also the push for international standardisation of units. This probably helped push terms inline with the original discoverers terms, or were “rebranded” to honour the original/significant discoverers (or for politics or whatever).

Some terms will still be used due to legacy, because it’s not worth trying to change it, or because the application is more inline with the original discovery.
The more modern terms probably refer to an improved design or different application of the same principle.
Eg a rheostat only has 2 terminals and works as a plain variable resistor, often built to handle higher power scenarios. A potentiometer uses 3 terminals and is often built for lower power scenarios.

While capacitor and condenser are the same thing, capacitor is likely more popular as it relates to it’s SI unit. And it’s SI unit describes what it does, It has capacity to store charge.

Things like valves Vs tubes. Tube describes what it is: a high vacuum tube. A valve describes what it does: varies the electricity flowing through it.
However, not all valves are vacuum tubes, some can be gas filled. And not all vacuum tubes are valves, for example television tubes.

A lot of this comes from multiple people developing similar things based on similar premises and applications, however all being slightly different. At the end of the day, the underlying physics is immutable, just the implementation and application changes.
And there was little-to-no communication between them, so people close to each discoverers would have used their implementation.
Sorta like Apple OSX Vs Windows. They are both desktop environment computers. But they do it in different ways to achieve the same result.
And then, all of these really useful technologies became standardised (or a clear “winner” emerged), and the terminology also became standardised.
All of which results in terms dropping out of favour

Cycles and condenser sound like something British would say. They also called tubes “valves.”

that was me sorry about that