About perfect fifths

I do not play guitar but I am enjoying your web site on theory. To
answer your question about the use of the word perfect the way I learned
it was this: if the upper note of an interval is found in the major
scale built on the lower note- it is known as a diatonic interval (C-G
diatonic 5th C-Ab not diatonic 6th). If the prime, fourth fifth or
octave are diatonic they are called perfect intervals. I hope this helps
Thanks