The secant of an angle is the reciprocal of the cosine of the angle.So the secant is not defined whenever the cosine is zeroThat is, whenever the angle is a multiple of 180 degrees (or pi radians).
Secant is a trignometric function. In a right triangle, the secant of an angle is the hypotenuse over the adjacent side. It is also the inverse of cosine. For example secant(x) = 1/cos(x)