In geometry, a centre (British English) or center (American English) (from Ancient Greek κέντρον (kéntron) 'pointy object') of an object is a point in some sense in the middle of the object. According to the specific definition of center taken into consideration, an object might have no center. If geometry is regarded as the study of isometry groups, then a center is a fixed point of all the isometries that move the object onto itself.
The center of a circle is the point equidistant from the points on the edge. Similarly the center of a sphere is the point equidistant from the points on the surface, and the center of a line segment is the midpoint of the two ends.
For objects with several symmetries, the center of symmetry is the point left unchanged by the symmetric actions. So the center of a square, rectangle, rhombus or parallelogram is where the diagonals intersect, this is (among other properties) the fixed point of rotational symmetries. Similarly the center of an ellipse or a hyperbola is where the axes intersect.
Main article: Triangle center
Several special points of a triangle are often described as triangle centers:
For an equilateral triangle, these are the same point, which lies at the intersection of the three axes of symmetry of the triangle, one third of the distance from its base to its apex.
A strict definition of a triangle center is a point whose trilinear coordinates are f(a,b,c) : f(b,c,a) : f(c,a,b) where f is a function of the lengths of the three sides of the triangle, a, b, c such that:
This strict definition excludes pairs of bicentric points such as the Brocard points (which are interchanged by a mirror-image reflection). As of 2020, the Encyclopedia of Triangle Centers lists over 39,000 different triangle centers.
A tangential polygon has each of its sides tangent to a particular circle, called the incircle or inscribed circle. The center of the incircle, called the incenter, can be considered a center of the polygon.
A cyclic polygon has each of its vertices on a particular circle, called the circumcircle or circumscribed circle. The center of the circumcircle, called the circumcenter, can be considered a center of the polygon.
If a polygon is both tangential and cyclic, it is called bicentric. (All triangles are bicentric, for example.) The incenter and circumcenter of a bicentric polygon are not in general the same point.
The center of a general polygon can be defined in several different ways. The "vertex centroid" comes from considering the polygon as being empty but having equal masses at its vertices. The "side centroid" comes from considering the sides to have constant mass per unit length. The usual center, called just the centroid (center of area) comes from considering the surface of the polygon as having constant density. These three points are in general not all the same point.
In projective geometry every line has a point at infinity or "figurative point" where it crosses all the lines that are parallel to it. The ellipse, parabola, and hyperbola of Euclidean geometry are called conics in projective geometry and may be constructed as Steiner conics from a projectivity that is not a perspectivity. A symmetry of the projective plane with a given conic relates every point or pole to a line called its polar. The concept of center in projective geometry uses this relation. The following assertions are from G. B. Halsted.