are, one might say, identified by what they are not: they are not abstract
, not multiply instantiated. (Though there is a potentially confusing piece of jargon that seems to contradict this: instances of properties are sometimes called abstract particulars
.) Hence, Socrates
is a particular (there's only one Socrates-the-teacher-of-Plato and one cannot make copies of him, e.g., by cloning him, without introducing new, distinct particulars). Redness, by contrast, is not a particular, because (it is held by metaphysical realists) it is abstract and multiply instantiated (my bicycle, this apple, and that girl's hair are all red).
Particulars might (or might not) be all individuals. At any rate, they are certainly all concrete--again, with the possible exception of abstract particulars (tropes).
The fact of the matter is that all such terms are used by philosophers with a rough-and-ready idea of how they work. If there is confusion or lack of agreement about the specifics, that is a reflection of the fact that philosophers have many competing metaphysical theories that inform more precise, but idiosyncratic, accounts of the meanings of these terms. Hence, for example, for convenience in formulating a solution to the problem of universals, 'particular' can be pressed into service in describing the particular instance of redness of a particular apple--even though redness (being abstract) is precisely the sort of thing that is not supposed to be particular. See philosophical jargon.