This subsection lists the definitions from Book 5 of "Euclid’s Elements". Euclid’s definitions avoid figures. At BookOfProofs, figures are given for the most definitions.

- Definition: 5.01: Magnitude is Aliquot Part
- Definition: 5.02: Multiple of a Real Number
- Definition: 5.03: Ratio
- Definition: 5.04: Having a Ratio
- Definition: 5.05: Having the Same Ratio
- Definition: 5.06: Proportional Magnitudes
- Definition: 5.07: Having a Greater Ratio
- Definition: 5.08: Proportion in Three Terms
- Definition: 5.09: Squared Ratio
- Definition: 5.10: Cubed Ratio
- Definition: 5.11: Corresponding Magnitudes
- Definition: 5.12: Alternate Ratio
- Definition: 5.13: Inverse Ratio
- Definition: 5.14: Composition of a Ratio
- Definition: 5.15: Separation of a Ratio
- Definition: 5.16: Conversion of a Ratio
- Definition: 5.17: Ratio ex Aequali
- Definition: 5.18: Perturbed Proportion