From wikipedia:
The perfect fifth or diapente is a musical interval which is responsible for the most consonant, or stable, harmony outside of the unison and octave.
The prefix perfect identifies it as belonging to the group of perfect intervals (Perfect fourth, Perfect octave) so called because of their extremely simple pitch relationships resulting in a high degree of consonance.
An example of a perfect fifth is:
C G
D A
F C
B F#
I pray this helps.
