I use the term "we" to represent "myself", to stay consistent with some standard sciences publication practices even though I am one person.
Most physics text books either do not directly define what physics is, they hint at it; or they give a definition like "physics is the science of measurement (or observation)" or "physics is the study of physical laws." For us physics is a combination of two things, both of which are required: observation and modeling. Something that is physical is observable. We define physics as the following:
Physics is the modeling of something physical.Or more briefly
Physics is studying physical models.Sometimes models are very abstract and apply to a group or class of physical observations.
Some models are better than others, in that they are simpler and/or give better agreements with observation. We refer to the fidelity of a model as how well it agrees with its corresponding observations. Higher fidelity models give better agreements to observations. In most cases the simplest model is the preferred so long as an excepted level of fidelity is achieved in using the model. Ultimately a high fidelity model will include the process of observation, but that is not required.
Models need not use mathematics to be expressed, but using mathematics is a goal. Only one of Newton's three physical laws is usually expressed as an equation, the other two are usually expressed as simple statements. Though all three laws can lead to equations when applied. As a young physics student I used to think that physic was trying to get models for what is "really" going on in nature. Now I know that that kind of thinking is not scientific, and leads to dogma. It does not matter what is really going on, what matters is how well models agree with what we observe.
When talking about quantum theory. Albert Einstein: God does not play dice. Niels Bohr: Who are you to tell god what to do.I like how Neils put Albert in his place. This tends to show that Neils shares my sentiment about what physics is. Newton and Einstein may have been very religious. I believe that at some level they knew how to separate dogma from science, but they may not have been conscious of it.
Unfortunately human languages need to be used to do physics and so terms come into physics from preexisting terms in the language used, and so the "physics terms" are automatically familiar to people who already knew the language before they know the terms used as a "physics term". Sometimes "physics terms" out grow there original non-physics meanings.
\(F = m a\) is considered a physical law. It's called Newton's 2nd law. It's undoubtedly the most popular physical law. Physical laws are simple abstractions that are commonly used to make models. They are generalized models. Many models are derived from physically laws, most models are derived from more general models, whither they be laws or not. Physical laws are not dogma, they just tend to work well in making models with good fidelity, for many applications. Physical laws are not required for making models. You can pull a model out of your but and just use it. Most physical laws, like Newton's 2nd, have been shown to be not completely consistent with all observations, so the use of the word law is a little misleading. Physical law classically implies that it is dogma, and that's not so. So the term Physical Law is misleading. Physics is not the study of dogma. Physics being a science, is always suspect. So when you see the term Physical Law, think of it as Well Accepted Generalized Physical Model, General Theory, or like term. The word Law in the term Physical Law comes from a legacy from in the time when top physicists like Issac Newton did not separate dogma from theory, such an idea was just to radical. Most people today cannot separate the two terms, but clearly there is a trend changing that.
In many cases the "speed of light" is the speed at which light travels in a vacuum or other media. In the nineteen hundreds it was found that the speed of light in a vacuum happens to be the same speed as any massless particles, and in fact that this speed was a much more fundamental universal constant. To label this speed as "speed of light" is misleading as it implies that the speed is specific to light when that is clearly not the case sometimes. Perhaps the term "universal limiting speed" is a better fit. But, you've got to use what catches on, "speed of light" is catchy. So, unfortunately, now the term Speed of Light has two distinct meanings in physics: Speed that light travels, and universal limiting speed.
Common examples of fundamental indefinables are time, length, and mass. Fundamental indefinables are observable, so they can be clearly be defined by observations, hence they are definable by defining how to observe them. The term is just nonsense that way. Using this kind of logic you can say that everything is indefinable, because all things are dependent on indefinables.
So we'd prefer the term fundamental physical measure, or fundamental measure in brief, as they are a set of physical measures that all other physical measures can be defined from. They're fundamental in that they can't be defined in terms of other fundamental physical measures. Fundamental measures are a matter of choice, but are generally chosen so as to keep models simple in form and/or follow excepted convention. We could replace the two fundamental physical measures time and length by frequency and velocity, but that would seem a little obfuscated.
The fact that we can't give as a precise definition for fundamental measures should not be emphasized in this term. What should be emphasized is that fundamental measures are independent of non-fundamental measures.
My biggest beef with the term fundamental indefinable is that it is the first term defined in most introductory physics text books which confuses the students at the start.