Thales theorem states that a diameter subtends a right angle to any point on a circle. Theorem is a special case of Inscribed Angle Theorem. Proof uses perpendicular bisector and triangle sum theorem
A secant is a line that intersects a curve at at least two distinct points. A chord is the line segment determined by the two points of a secant. A straight line can intersect a circle at zero, one, or two points
Tangent touches a circle at one point, forming a right angle with radius. Secant intersects a circle at two points, passing through its interior