In geometry, a centre (British English) or center (American English); () 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. Circles, spheres, and segments 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. Symmetric objects 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. Triangles Several special points of a triangle are often described as triangle centers: the circumcenter, which is the center of the circle that passes through all three vertices; the centroid or center of mass, the point on which the triangle would balance if it had uniform density; the incenter, the center of the circle that is internally tangent to all three sides of the triangle; the orthocenter, the intersection of the triangle's three altitudes; and the nine-point center, the center of the circle that passes through nine key points of the triangle. 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: f is homogeneous in a, b, c; i.e., f(ta,tb,tc)=thf(a,b,c) for some real power h; thus the position of a center is independent of scale. f is symmetric in its last two arguments; i.e., f(a,b,c)= f(a,c,b); thus position of a center in a mirror-image triangle is the mirror-image of its position in the original triangle. This strict definition