Radius: A line segment drawn joining the centre and the boundary line (known as circumference) is called the Radius of the circle.
Diameter: A line segment crossing the circle passing through the centre of the circle.
Circumference: The boundary or the perimeter of the circle.
Chord: A line segment joining any two points on the circumference of the circle.
Arc: Part of the circumference of the circle.
Tangent: A line that touches the circumference of the circle at exactly one point.
Sector: An area enclosed by two radii of a circle.

Basic formulae of a circle:
circumference = \(2*pi*r\), where \(r\) is the radius of the circle.
area = \(pi*r^2\)
diameter = \(2*r\)

Properties of Arcs and angles :
The angle made by the ends of the arc at the centre of the circle is called its angle.

• Equal arcs subtend equal angles at the centre of the circle and vice versa.
• The angle at the centre od the circle is twice the angle at the circumference subtended by the same arc.

Properties of Chords and tangents:

• Diameter is the longest chord of a circle.
• A perpendicular line from the centre to the chord bisects the chord.
• Equal chords are equidistant from the centre of the circle.
• Equal chords subtend equal angles at the centre of the circle.

• A tangent to a circle is perpendicular to the radius drawn at the point of contact.
• Tangents to a circle from an exterior point are equal.
• The angle between the tangent and the chord is equal to the inscribed angle on the opposite side of the chord.

Basic properties of a circle

• Angle in a semicircle is the right angle.
• Angle in the same segment of a circle are equal.