From the centre out

A circle is the set of all points the same distance from one fixed point. That fixed point is the centre, usually labelled O.

• a radius is a straight line from the centre out to the edge — its length is that fixed distance;
• a diameter is a straight line right across the circle through the centre. It is made of two radii end to end, so it is twice the radius: d = 2r;
• a chord is any straight line joining two points on the edge (a diameter is the longest chord);
• the circumference is the distance all the way around the edge.

Pieces of the circle

Once we can name the edge and the centre, we can name the pieces too:

• an arc is part of the edge — a curved piece of the circumference. The shorter one is the minor arc, the longer one the major arc;
• a sector is the "pizza slice" between two radii (the region enclosed by two radii and the arc between them);
• a segment is the region cut off by a chord — the piece between a chord and the arc;
• a tangent is a straight line that just touches the circle at exactly one point, without crossing it.

See them on one circle

Step through the figure to add one part at a time — the centre, a radius, the diameter, a chord, a sector and finally a tangent.