In metaphysics, particulars are, one might say, identified by what they are not: they are not abstractions, not multiply-instantiated--i.e. they are concrete. (There are, however, theories of abstract particulars or tropes.) 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 woman's hair are all red).

Sybil Wolfram writes

Particulars include only individuals of a certain kind: as a first approximation individuals with a definite place in space and time, such as persons and material objects or events, or which must be identified through such individuals, like smiles or thoughts.

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.