This subsection lists the definitions from Book 3 of "Euclid’s Elements". Euclid’s definitions avoid figures. At BookOfProofs, figures are given for the most definitions.

- Definition: 3.01: Congruent Circles
- Definition: 3.02: Tangent to the Circle, Straight Line Touching The Circle
- Definition: 3.03: Circles Touching One Another
- Definition: 3.04: Chords Equally Far From the Center of a Circle
- Definition: 3.05: Chords Being Further from the Center of a Circle
- Definition: 3.06: Segment of a Circle, Arc
- Definition: 3.07: Angle of a Segment
- Definition: 3.08: Angle in the Segment (Inscribed Angle)
- Definition: 3.09: Angle Standing Upon An Arc
- Definition: 3.10: Circular Sector, Central Angle
- Definition: 3.11: Similar Circular Segments