Is there a generally accepted philatelic definition of the term "rare". We all know that some ebay sellers optimistically describe stamps catalogued at $1 or less as rare.
I suppose catalogue value is not a true measure of rarity, because that combines factors including scarcity, demand for the stamp in question, etc.
Talk to TonyMacG on the Uglies thread, and he'll give you a list of Indian States stamps that were only issued in a few dozen, yet may sell for only a few hundred dollars.
An item might be rare because not many were issued and/or still exist, but that doesn't make it valuable. Conversely, something expensive may not be rare, if its popular. There's no shortage of 5 bob Bridges as Norm pointed out, or $5 Columbians, or 5 Pound orange QV, yet those all sell for thousands.
The "All Country is Red" stamp from China makes $30-50,000 USD at auction, and a typical sale in HK will have anywhere from 2 to a record 7 examples in 1 auction.
Picasso paintings sell for millions. Are Picasso's "rare"?
When demand is factored into the equation, the term "rare" gets completely skewed.
I'd say a stamp worth $100-1,000 is a 'good' stamp; $1000+ is 'scarce'...maybe; $10,000+ and you're getting into rare...maybe. What about auctions described being a sale of "fine stamps and covers"; fine is an adjective often used in auction lingo to mean "expensive".