Perfect pitch, also known as absolute pitch, is the ability to identify or sing a given note without reference. In other words, if I were to play an E (and only an E) on the piano, someone with perfect pitch would be able to identify the note blind folded. A singer with perfect pitch could also accurately sing that E before I played it.
Perfect Pitch is not Relative Pitch
Relative pitch is the ability to identify or sing a given note by using a reference note. In other words, if I know what a C sounds like, I can identify an E because I know an E is a major third above a C (although, in reality, my relative pitch is quite poor).
It is not uncommon to get perfect pitch and relative pitch confused. They are, however, two very different things. Relative pitch is all about intervals. Relative pitch is about knowing what a perfect fifth sounds like, or a major third. It can also be used to identify different types of chords, such as major, minor or major seventh. This is was is usually taught in colleges, and what I was taught in my high school music theory class.
Perfect pitch depends on color hearing. I can sing an A not because I am comparing it to another note, but because I know what an A sounds like, as if it has its own personality or “color”. I don’t need to hear a C to know what an A is, for perfect pitch does not require comparison (though it is very helpful to compare when training). The only course that I know of is the one made by David Lucas Burge.