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
Algebraic geometry points of algebraic geometry. The fact that the field of the real numbers is an ordered field cannot be ignored in such a study. For example, the curve of equation formula_8 is a circle if formula_9, but does not have any real point if formula_10. It follows that real algebraic geometry is not only the study of the real algebraic varieties, but has been generalized to the study of the "semi-algebraic sets", which are the solutions of systems of polynomial equations and polynomial inequalities. For example, a branch of the hyperbola of equation formula_11 is not an algebraic variety, but