Various terminologies pertaining to measurements

Solution 1:

4. The thing being measured
5. The thing undergoing the measurement

What are good, concise names for 4 and 5?

The thing undergoing the measurement is normally the subject:

[Merriam-Webster]
3 c(1) : one that is acted on
             // the helpless subject of their cruelty
3 c(2) : an individual whose reactions or responses are studied

Hence:

• The subject of the test.
• The subject of the experiment.
• The subject of the measurement.

For the thing being measured, I would use parameter:

[Merriam-Webster]
2 : any of a set of physical properties whose values determine the characteristics or behavior of something
    // parameters of the atmosphere such as temperature, pressure, and density

In experiments, you normally set parameters in order to produce a result.

But in this case, you are starting off with a result and working backwards—measuring the existing unknown parameters that have produced the result.

We normally refer to the name of a specific parameter when measuring it (temperature, pressure, and density, to use those from the example sentence in the definition), but parameter is the umbrella term for those things.