The error is in equating the definition with the concept - the definition focusses on essentials , while the concept subsumes all characterstics of all real chairs. The metaphysical idealists claim that concepts exist as entities in reality - perfect chairs, perfect circles, perfect essences. They claim that our failure to perceive these perfect entities is due to the imperfect nature of our senses. Objectivism recognizes that concepts are mental entities, not material entities. Just because I can imagine a six-headed unicorn doesn't mean that one exists.
Existence is primary, consciousness is secondary.
The epistemological idealists claim that while concepts are mental entities and not physical entities, they are arbitrary and therefore powerless to deal with reality. Objectivism acknowledges that man has volition and may form concepts arbitrarily if he so chooses. But properly-formed concepts are derived from reality, and not arbitrary. Such concepts derived from reality are the only valid concepts. In order to perceive characterstics, common or otherwise, they must exist in reality.