Abstract and Concrete
The words "abstract" and "concrete" are used loosely in everyday speech. Concrete terms are logically those terms which refer to sensible particulars. A particular is an individual that is of a certain kind; and being of a certain kind is a member of a class. Sensible particulars are apprehended at once by sense-perception and by the intellect unless we are conceptually blind. Then these are apprehended only by the senses.
Such conceptual blindness occurs when the sensible individual thing is apprehended by one sphere of sense-perception and not by another, as when that which is perceived is not understood at all. For example, a person who is conceptually blind in his or her sense of touch may be able to identify the object by the sense of smell. This happens when a person cannot identify the kind of object it is by touching it, but can do so by smelling it. When it is just touched by this person, the individual sensible thing is a raw individual, perhaps in some way familiar, as having been touched before, but without an identity, a name.
When we are not conceptually blind, the terms "flower" or "pencil" name certain things that are both perceived and also understood as being of a certain kind. Human apprehension differs radically from the purely sensible apprehension of brute animals that do not have intellects. For them, the world consists of raw individuals. We cannot imagine how the world of sensible object appears to them.
In human apprehension, which is both sensitive and intellectual, the abstract object of thought is one that cannot be instantiated. We call the object of thought abstract if we cannot give particular instance of it that are sensible perceptually instantiated. They are, therefore, abstract in their referential significance.
In short concrete terms are those which can be perceptually exemplified or instantiated: abstract terms are those which cannot be perceptually exemplified or instantiated. They refer to objects that are purely objects of conceptual thought.