Secant line a circle in the Euclidean plane, a line joining two of them would be a 2-secant (or "bisecant") and a line passing through only one of them would be a 1-secant (or "unisecant"). A unisecant in this example need not be a tangent line to the circle. This terminology is often used in incidence geometry and discrete geometry. For instance, the Sylvester–Gallai theorem of incidence geometry states that if points of Euclidean geometry are not collinear then there must exist a 2-secant of them. And the original orchard-planting problem of discrete geometry asks for a bound on the number of
Line (geometry) ray, the "AD" ray is called the "opposite ray". Thus, we would say that two different points, "A" and "B", define a line and a decomposition of this line into the disjoint union of an open segment and two rays, "BC" and "AD" (the point "D" is not drawn in the diagram, but is to the left of "A" on the line "AB"). These are not opposite rays since they have different initial points. In Euclidean geometry two rays with a common endpoint form an angle. The definition of a ray depends upon the notion of betweenness for points on