Thing: Whatever can be counted and thought of as one and distinguished from whatever else one can think of.
Thus, what is a thing depends on thinking and distinguishing, which seems sensible in as much as there are many things one can think of that are not real or - even more puzzling - that may not be real.
It should also be carefully noted that what is called a thing here is individual insofar as it can be distinguished from anything else, but that this does not mean that it is concrete or a particular: As defined, the class of complex numbers or the class of integers are things as well as any one complex number or any one integer, though the classes are not of the same kind as their elements, and contain many elements even while counting as one class.
Likewise, countries are things, and abstract ideas, like the differential of x2 and 3-dimensional space count as things.
In short, it seems as if the term and idea of a thing are quite fundamental to human thought, and it is a mistake to restrict the term to such entities as are particular, concrete or individual and also a mistake to restrict the term to such entities as exist.
See also: Entity