Rarity was a very common term used by Club Penguin players, and sometimes even by moderators and in promotional material, which generally referred to items of any kind that few people that still played Club Penguin owned, and is thus "rare" to see.
What was considered "rare" varied heavily between players, and thus had no conclusive definition. For instance, some considered items that have not returned in many years, such as since 2006 or 2007 to be "rare", while others consider items available to a limited group of players, such as the Penguin Of The Week Background to be "rare" as well. Pins and anniversary hats were widely considered "rare", as, with few exceptions for the former, were never rereleased after their initial availability.
For purposes of the Club Penguin Wiki, items were marked as "rare" if they were not available for over four years before the current day, and therefore, the items listed on this page are items that were last available on March 29, 2013 or before. This specific definition, like the ones listed above, are not official, and should not be taken as such.