Lets take a bunch or particles. You put some energy in the ball by throwing it. It now has kinetic energy. Each particle has kinetic neergy, but that movemnt is ordered, every molecole it traveling in a similar way (in average). Now put teh same energy in the ball by slapping it a couple of times. It still lies on a table, but not perticles inside moves faster. In unrdered, chaotoc way (they were moving like that before, bot noq thwy are moving a bit more). energy of that chaotic movement (both kinetic and potential*) is the internal energy. In gas this is mainly kinetic energy of movement (in hotter gases rotational energy and energy of vabratons in the molecule join, only then "for quantum reasons"). Now you can estimate entropy (count "microstates" the system can be while keeping the same "macro" state - mainly the same energy. Take logaritm of it. Then "add" some more energy, couns states, you get the new entropy. From both you get the relation between S and U in this system, so temperature.
But you can see we separate the ordered movement of the center of the mass from the "thermal"/chaotic movements. A cold satelite orbiting Earth would be quite considered hot is we include the kinetic energy - and if the satelite would be captured by atmosphere, it would become quite boiling, when the kinetic energy oh the satelite become the thermal energy of the satelite. The exact definition may be tricky (think about turbulence, where it is no longer a chaotic movement o fluid and it is a thermal chaotic movement or perticles?), but for now it ie snouhg to get a small portion of mass, transfer to it's canter of mass reference frame, and then conut the total energy.

As you see, one particle is problematic. It doen't have internal energy (as long as we doen't conut for example electrons on different "orbitals"), it is always staying still in respect to its centre of mass (duh). If we take "one articla in a box" the volume of aviable microstates is always the same (it would required another paragraph, jsut trust me). So, we change energy, entropy is the same. So, acording to the definition the temperature is infinite. You can think of that particle and its exitaction levels (that will be interacting with surronding radaition), or some small quantum systems where entropy (so, temperature) can be introducted. But those are exceptions, thikn of a temperature anmd tehrmal energy as a property of a big systems.


*) Kinetic is obvious, more energy, it moves vaster in average. It may be a straight motion like i gases, or wibration in solids. Potential: lats say you have two particles with a bond. In an equlibrium, they lenght of the bond will be at minimum. But it molecuels vibrate, most of the time they will be at different distance - more energy in the bond. Far from qiantum effect, thanks to the equipartition theorem, each of the degree of freedom has the same portion of energy. Kinetic and potential part are considered different dimensions. Each get mean energy (per perticle) 1/2kT