What are concrete particulars in a philosophical sense?

Sensory evidence.

First, they are epistemological in nature.

Second, particulars are the species of abstractions, which are the genus. But each genus is the species of the next genus, and as a species it in itself becomes a particular.

"Abstractions as such do not exist: they are merely man’s epistemological method of perceiving that which exists—and that which exists is concrete...

"Man retains his concepts by means of language. With the exception of proper names, every word we use is a concept that stands for an unlimited number of concretes of a certain kind. A concept is like a mathematical series of specifically defined units, going off in both directions, open at both ends and including all units of that particular kind. For instance, the concept “man” includes all men who live at present, who have ever lived or will ever live—a number of men so great that one would not be able to perceive them all visually, let alone to study them or discover anything about them...

"A concept is a mental integration of two or more units which are isolated by a process of abstraction and united by a specific definition. By organizing his perceptual material into concepts, and his concepts into wider and still wider concepts, man is able to grasp and retain, to identify and integrate an unlimited amount of knowledge, a knowledge extending beyond the immediate concretes of any given, immediate moment." Ayn Rand: "Introduction to Objectivist Epistemology"

Hello,

Anything that is objectively real and pertinent to the discussion is a concrete particular..

Thank you for your question.