• Circle - definition
• Parts of a circle - a pictorial index
• Radius
• Diameter
• Circumference
• Pi
• Area of a circle
• Annulus
• Area of an Annulus
• Chord
• Tangent
• Secant
• Intersecting Secant Lengths Theorem
• Intersecting Secant Angles Theorem
• Intersecting Chords Theorem
• Concentric circles
• Incircles
• Circumcircles

• Semicircle
• Sector of a Circle
• Area of a Sector
• Segment of a circle
• Area of a Segment given central angle
• Area of a Segment given segment height

• Inscribed Angle
• Central Angle
• Central Angle theorem
• Thales' Theorem
• Angle inscribed in a semicircle

• Arc definition
• Major and Minor Arcs
• Arc length
• Angle measure of an arc
• Adjacent arcs
• Intercepted Arc
• Radius of an Arc or Segment
• Sagitta - Height of an Arc or Segment

• Finding the center with compass and ruler
• Finding the center with any right-angled object

• See 'Equations of a circle' Section

• See Ellipse Section