The interval which encompasses 5 degrees of a diatonic scale. It is usually close in size to the frequency ratio 3:2, which is approximately 702 cents.
In the usual 12-edo tuning, the perfect 5th encompasses 7 semitones, and is thus exactly 700 cents.
Below is a graph of the size of the best approximation to the 3:2 "5th" in cents, for all the EDOs from 10 to 72:
Below is a table showing the sizes of "perfect-5th" for various tunings of the historically important meantone family:
tuning ~cents of generator 12-edo 700.000000 1/11-comma 699.9998836 103-edo 699.029126 91-edo 698.901099 79-edo 698.734177 67-edo 698.507463 1/6-comma 698.3706193 55-edo 698.181818 98-edo 697.959184 43-edo 697.674419 1/5-comma 697.6537429 3/14-comma 697.3465102 74-edo 697.297297 2/9-comma 697.1758254 105-edo 697.142857 31-edo 696.774194 1/4-comma 696.5784285 112-edo 696.428571 81-edo 696.296296 7/26-comma 696.164846 50-edo 696.000000 5/18-comma 695.9810315 2/7-comma 695.8103467 69-edo 695.652174 88-edo 695.454545 107-edo 695.327103 1/3-comma 694.7862377 19-edo 694.736842