A line that "just touches" the circle as it passes by is called a Tangent. A line segment that goes from one point to another on the circle's circumference is called a Chord. If it passes through the center it is called a Diameter. And a part of the circumference is called an Arc.
A central angle is an angle whose apex (vertex) is the center O of a circle and whose legs (sides) are radii intersecting the circle in two distinct points A and B. Central angles are subtended by an arc between those two points, and the arc length is the central angle of a circle of radius one (measured in radians).