"Conceptual Common Denominator" = a similarity
The "Conceptual Common Denominator" or "CCD" is surely one of the wooliest terms in the vast sheep station of Objectivist jargon, and is central to Rand's much hyped theory of Concept Formation. However, shorn of its typical verbiage, it merely means something one object has in common with another, or a similarity.
The problem is, of course, that Rand's theory is supposed to solve the age-old problem of universals, or why different things are similar. Her answer appears to be because they have one or more "Conceptual Common Denominators" ie: similarities. So, in classic Randian style, her astonishing answer to the problem of why different things are similar, is...because they have similarities!
William Thomas of The Objectivist Centre is a brave man. Towards the end of this short summary of Rand's theory, he makes a rare effort to actually give some specific examples of Rand's "CCDs." I will leave it to readers to judge the level of profundity the application of Rand's insight to reality reveals.
The is a partial listing of some of the Conceptual Common Denominators Rand proposes (many of these are implicit in her presentations of definitions or distinguishing characteristics. Where I could not clearly determine to my satisfaction what the CCD might be, I have included the distinguishing characteristics Rand mentions." (sic)
(The page references are from the "Introduction to Objectivist Epistemology")