Circle terms

Center: The center of a circle is the point from which all points on the circle are at an equal distance.
Radius: The distance from the center of a circle to any point on it is called its radius.
Chord: A straight line joining any two points on a circle is called a chord.
Diameter: The chord of a circle passing through its center is called its diameter. The diameter is the longest chord of a circle.
Circumference: The perimeter of a circle or the length of the circle's boundary.

Arc: A part of the circle's boundary starting from one point and ending at another is called an arc.
Central angle: The angle subtended (made) by an arc on the center of the circle is called the central angle.

Sector: A part of a circle enclosed by two radii and an arc of the circle is called a sector. 
Minor and major sectors: Any arc and its two corresponding radii form two sectors, as shown in the image above. The smaller one is called the minor sector and the larger one is called the major sector.
Section: Any chord drawn on a circle divides it into two sections. Thus, the part of a circle enclosed between a chord and the corresponding arc is called a section.
Major and minor sections: Since the chord of a circle divides it into two parts (equal or unequal), this gives rise to two sections. The larger section is called the major section while the smaller one is called the minor section.
Semi circle: The section of a circle formed by its diameter is called a semi circle.

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