The term “condition rarity” is a numismatic term that refers to a coin that, while is not in and of itself rare (which would be an “absolute rarity”), is in a grade or state of preservation that makes it rare.
In other words, a condition rarity is an item that is rare in high grades, whereas an absolute rarity is rare in ANY grade.
Philatelically speaking, a U.S. C3a, 1869 invert, or Pan Am invert would be an absolute rarity, and a common 19th century stamp graded a 100J would be a condition rarity.
This past weekend at CHICAGOPEX I picked up a stamp that I believe falls into the category of condition rarity. R21c, the 4-cent Playing Cards, while not itself very rare (it catalogs $800; I have about a half dozen examples), is almost always found either faulty or very poorly centered.
Sound examples with nice centering come around… well, almost never.