Editing Circle (section)

==Terminology==
* [[Annulus (mathematics)|Annulus]]: a ring-shaped object, the region bounded by two [[concentric]] circles.
* [[Circular arc|Arc]]: any [[Connected space|connected]] part of a circle. Specifying two end points of an arc and a centre allows for two arcs that together make up a full circle.
* [[Centre (geometry)|Centre]]: the point equidistant from all points on the circle.
* [[Chord (geometry)|Chord]]: a line segment whose endpoints lie on the circle, thus dividing a circle into two segments.
* [[Circumference]]: the [[length]] of one circuit along the circle, or the distance around the circle.
* [[Diameter]]: a line segment whose endpoints lie on the circle and that passes through the centre; or the length of such a line segment. This is the largest distance between any two points on the circle. It is a special case of a chord, namely the longest chord for a given circle, and its length is twice the length of a radius.
* [[Disk (mathematics)|Disc]]: the region of the plane bounded by a circle. In strict mathematical usage, a circle is only the boundary of the disc (or disk), while in everyday use the term "circle" may also refer to a disc.
* [[Lens (geometry)|Lens]]: the region common to (the intersection of) two overlapping discs.
* [[Radius]]: a line segment joining the centre of a circle with any single point on the circle itself; or the length of such a segment, which is half (the length of) a diameter. Usually, the radius is denoted <math>r</math> and required to be a positive number. A circle with <math>r=0</math> is a [[Degeneracy (mathematics)|degenerate case]] consisting of a single point.
* [[Circular sector|Sector]]: a region bounded by two radii of equal length with a common centre and either of the two possible arcs, determined by this centre and the endpoints of the radii.
* [[Circular segment|Segment]]: a region bounded by a chord and one of the arcs connecting the chord's endpoints. The length of the chord imposes a lower boundary on the diameter of possible arcs. Sometimes the term ''segment'' is used only for regions not containing the centre of the circle to which their arc belongs.
* [[Secant line|Secant]]: an extended chord, a coplanar straight line, intersecting a circle in two points.
* [[Semicircle]]: one of the two possible arcs determined by the endpoints of a diameter, taking its midpoint as centre. In non-technical common usage it may mean the interior of the two-dimensional region bounded by a diameter and one of its arcs, that is technically called a half-disc. A half-disc is a special case of a segment, namely the largest one.
* [[Tangent]]: a coplanar straight line that has one single point in common with a circle ("touches the circle at this point").
All of the specified regions may be considered as ''open'', that is, not containing their boundaries, or as ''closed'', including their respective boundaries.
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{| class="skin-invert-image" style="float:left;" cellspacing="0" cellpadding="0"
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|[[File:CIRCLE LINES.svg|right|thumb|Chord, secant, tangent, radius, and diameter]]
|[[File:Circle slices (mul).svg|right|thumb|Arc, sector, and segment]]
|}
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