What is Great circle, spherical triangle?

A great circle of a sphere is its intersection with a plane through its centre; any other plane section is a small circle. Through two points that are not diametrically opposite passes exactly one great circle, and the shorter of its two arcs is the shortest path on the sphere between the points. A spherical triangle is the figure formed by three points of the sphere, its vertices $A$, $B$, $C$, joined by arcs of great circles of less than $180{}^{\circ}$. Its sides $a$, $b$, $c$ (opposite $A$, $B$, $C$) are measured by the angles they subtend at the centre; its angles $A$, $B$, $C$ are the angles between the planes of the sides that meet at each vertex.