> The meaning of perfect pitch is unambiguous. When a note is played someone with perfect pitch can identify that pitch (Ab, F, C#, etc) without a reference. Also known as absolute pitch.

How does that work with variations in concert pitch? As I understand it, although the most common standard is to tune the A above middle C to 440 Hz, it's not universal.

Are the differences too small to matter? Or does music played on instruments tuned to a different standard sound "off"? Or are the other standards sufficiently rare that most musicians will never encounter them?

The definition of relative pitch is unfortunately ambiguous, however--I've heard it defined as being able to pick out one note or several note, but having to rely on that reference note from memory to determine the pitch of other notes. I have A flat and E flat burned into my mind from playing Scott Joplin's Maple Leaf rag so many times as a teenager, so if I think hard I can figure out what any note is based on that reference. I can't quickly identify a note without doing that in my mind though, so I don't claim to have perfect pitch.