Knowing the correct vocabulary for parts of a circle is essential — questions regularly ask you to name, identify, or use specific parts.
Key terms:
- Centre — the fixed point equidistant from every point on the circle
- Radius — a straight line from the centre to the circumference (plural: radii)
- Diameter — a chord that passes through the centre; always twice the radius
- Circumference — the perimeter of the circle ($C = \pi d = 2\pi r$)
- Chord — a straight line joining any two points on the circumference (not necessarily through the centre)
- Arc — a portion of the circumference between two points
- Sector — a "slice" of the circle bounded by two radii and an arc (like a pie slice)
- Segment — the region between a chord and the arc it cuts off
- Tangent — a straight line that touches the circumference at exactly one point; always perpendicular to the radius at that point
Common error: confusing a sector (bounded by two radii) with a segment (bounded by a chord and an arc).