Angle
In geometry, an
angle is the figure formed by two rays or line segments, called the
sides of the angle, sharing a common endpoint, called the
vertex of the angle. Angles are usually presumed to be in a Euclidean plane or in the Euclidean space, but are also defined in non-Euclidean geometries. In particular, in spherical geometry, the spherical angles are defined, using arcs of great circles instead of rays.
Angle is also used to designate the measure of an angle or of a rotation. This measure is the ratio of the length of a circular arc to its radius. In the case of a geometric angle, the arc is centered at the vertex and delimited by the sides. In the case of a rotation, the arc is centered at the center of the rotation and delimited by any other point and its image by the rotation. The word
angle comes from the Latin word
angulus, meaning "a corner". The word
angulus is a diminutive, of which the primitive form,
angus, does not occur in Latin. Cognate words are the Greek ἀγκύλος
, meaning "crooked, curved," and the English word "ankle".