A diatonic interval of five notes. Its ratio is 2:3, the diapente of the ancients. [A Dictionary of Musical Terms]
The interval between the root and the fifth tone above it.
If it be asked why no more primes than 2, 3, and 5 are admitted into musical ratios, one reason is that consonances whose vibrations are in ratios whose terms involve 7, 11, 13, etc., would be less simple and harmonious than those whose terms involve the lesser primes only. Another reason is this - as perfect fifths and other intervals resulting from the number 3 make the schism of a comma with perfect thirds and other intervals resulting from the number 5, so intervals resulting from the numbers 7, 11, 13, etc., would make other schisms with both those kinds of intervals. [Scientific Basis and Build of Music, page 75]
Table 11.02 - Fifth is Double Tone
7.14 - Fifth
9.11 - Love or Sympathy is Perfect Continuity
11.13 - Dominant Conditions are Mated Opposing Pairs as Fifths