Given that A circle is shown with the center O. OR is a segment joining center O to a point R on the circle. PQ is a line touching the circle in Point R. Line MN is shown intersecting the circle in points S and T.
Since given that PQ is a line touching the circle in point R we have points P, Q and R are collinear.
PR, PQ or RQ is the tangent to the circle
MN is not a tangent since MN intersects the circle at two points S and T.
A tangent will not cut the circle at two points but meet the circle at only one point.