The word "abstract" is interesting.
Conservatives critique lefties of having "abstract" thinking, which they say means out of touch with reality. The dictionary definition of "abstract" said it means "Considered apart from concrete existence."
But the word "abstract" essentially means to draw out something, usually to draw out the essential characteristics of something. When one writes an "abstract" in a lab report they draw out the important parts of the lab report and put them in a summary statement at the beginning.
Epistemologically, to “abstract” means to draw out the essential parts of reality. It means to identify the essential, relevant, and important trends of something and use them as an intellectual to make decisions with them. Abstraction takes mounds of data and simplifies it so the human brain can understand it. Abstraction highlights the important trends of something; rather than mindlessly looking at streams of data.
Abstraction is the process one engages in when they form concepts. A concept is a "mental integration of two or more units with the same distinguishing characteristic." (Introduction to Objectivist Epistemology). Man takes the streaming data all around him and simplifies it into concepts by drawing out the essential characteristics of something and forming a concept about it. As a simple example, he sees several cats in his daily life. Instead of memorizing them all as a new odd phenomena, he draws out those things that make them similar and identifies them as a "cat."
The one part of the original definition of abstract that is correct is that abstractions are held mentally; they are not held in existence. The "abstraction" is what the human does to simplify it in his mind. What actually exists is not a "cat" but the individual cats that exist. This is the only way that "abstract" can be considered "apart from existence."
Similar to a corporate executive, who reads an "abstract" of a lab report (and usually nothing else) and then has a simplified conclusion about the entire report thus liberating his mind and time; a "concept" also liberates man's mind and time by giving him a simplified conclusion about the reality about him in a bundled packet. A "concept" of something, retrieved by a method of abstraction, represents a large amount of data but is in effect turned into the equivalent of a (simplified) percept (an automatically integrated sensation seen by man's brain) but is not actually a percept. "Concepts" liberate man's mind in that man, after he develops it, can then identify all existents of the same kind after he develops said concept.
"Abstractions" are held mentally by a human, but they are not out of touch with "concrete existence." Abstractions, done properly, should be concepts based upon the reality around us.