- The ratio Circumference/diameter is a constant for any circle.
- A
**tangent**to a circle is a straight line that meets the circle in one point only and does not cut the circle when produced.*AC*in Fig.-1 is a tangent to the circle since it touches the curve at point*B*only. If radius*OB*is drawn, then angle*ABO*is a right angle.

- A
**sector**of a circle is the part of a circle between radii (for example, the portion*OXY*of Fig.-2 is a sector). If a sector is less than a semicircle it is called a**minor sector**, if greater than a semicircle it is called a**major sector**.

- A
**chord**of a circle is any straight line that divides the circle into two parts and is terminated at each end by the circumference.*ST*, in Fig.-2 is a chord. - A
**segment**is the name given to the parts into which a circle is divided by a chord. If the segment is less than a semicircle it is called a**minor segment**(see shaded area in Fig.-2). If the segment is greater than a semicircle it is called a**major segment**(see the unshaded area in Fig.-2). - An
**arc**is a portion of the circumference of a circle. The distance*SRT*in Fig.-2 is called a**minor arc**and the distance*SXYT*is called a**major arc**.