Where, exactly, does the boundary lie between 'entangled' particles and merely 'interacting' or 'coupled' ones? Have scientists discovered, yet, the precise amount of interaction needed to actually 'entangle' a pair or more of particles, beyond mere interaction or even coupling?
Is there a distinct difference, or is it merely a matter of degree?
 A: Let us begin by examining bar magnets (with some distance between them). Once they are aligned with each other, they are in a coupled state for exactly as long as no other external interaction disturbs their common state. Our measuring instruments, with which we can see the coupling, do not destroy this coupling. The simplest measuring device is simply our eyes.
Now we want to align the bar magnets in a black box. The alignment, which is to be random in the experiment, is not visible to us. If we place a small bar magnet on the top of the box, this magnet will move and be aligned by the magnets inside the box. We are able to find the current positions of the magnets in the box.
Now we are reducing more and more the dimensions of the two bar magnets in the box. At a moment, the measuring magnet on the black box (being „stronger“) will rotate the magnets in the box. The experiment will be successful as long as the measuring magnet on the box will be smaller the magnets in the box.
And now we have two electrons in the box (let's assume we can hold them with optical tweezers). And through their magnetic dipole moments, they align each other, just like the bar magnets used before. Do we at least have the possibility to see that they are in a coupled state?
Perhaps a moving electron on the top of the box is deflected by the electrons in the box under the influence of the magnetic field. However, the electron on the top could influence the electrons in the box and bring them out of alignment. However, by running the experiment several times, we can statistically prove that the two electrons in the box are aligned.
This is a contrived example. In principle, however, experiments with entangled photons (Spontaneous parametric down-conversion) work in exactly the same way. Only statistically can we prove that the photons generated in this way are entangled.

Source
Look at point 2 in the sketch. We believe that the particles are in a common intermediate state described by a common function. Does this mean that when one of the particles is measured (which is only statistically possible over several pairs of particles!) the second particle is transferred from the common state to its individual state? Or does only our non-knowledge about the actual states collapse?
Conclusion
The boundary between "entangled" particles and merely "interacting" or "coupled" particles is thus the possibility of measuring their states without destroying their states (coupled particles) or not having this possibility without statistical methods (entangled particles).
