Circle Terminology
Circle Terminology
Precise vocabulary is essential for circle geometry. Every term names a specific geometric object, and using the wrong term in a proof or explanation introduces ambiguity. Learn the definitions — not just the words.
Definitions
| Term | Definition |
|---|---|
| Centre | The fixed point equidistant from all points on the circle |
| Radius | A line segment from the centre to any point on the circle |
| Diameter | A chord passing through the centre; equal to twice the radius |
| Chord | A line segment joining any two points on the circle |
| Arc | A portion of the circumference; minor arc is the shorter, major arc is the longer |
| Sector | The region bounded by two radii and an arc (like a pizza slice) |
| Segment | The region bounded by a chord and an arc (minor or major) |
| Tangent | A line that touches the circle at exactly one point; perpendicular to the radius at that point |
| Circumference | The perimeter of the circle; \( C = 2\pi r = \pi d \) |
Key Takeaways
- Diameter = 2 × radius. The diameter is the longest chord.
- A tangent meets the circle at exactly one point and is perpendicular to the radius at that point.
- Sector = two radii + arc. Segment = chord + arc.
- Minor arc/sector/segment: smaller; major: larger.
- Circumference: \( C=\pi d = 2\pi r \). Area: \( A=\pi r^2 \).