Parts of a Circle
Circumference
The perimetre. All the way round the outside of the circle.
Arc
Part of the outside of the circle.
Radius
A line from the centre of the circle to the edge.
Diameter
Two times the radius.
Tagent
A line that meets the edge of the circle once.
Chord
A line that goes from one edge of the circle to the another edge.
Semi-circle
Half of a circle.
Sector
The area inside an arc and two radius.
Segment
The area between an arc and a chord.