Given two points A and B, lines from them to center of the circle form the central angle ∠AOB. The central angle is the smaller of the two at the center. It does not mean the reflex angle ∠AOB. As you drag the points above, the angle will change to reflect this as it increases through 180°
The two points A and B can be isolated points, or they could be the end points of an arc or chord. When they are the end points of an arc, the angle is sometimes called the "arc central angle".
A similar concept is the inscribed angle. This is the angle subtended at a point on the circle by the two given points.
See Inscribed Angle definition
The central angle is always twice the inscribed angle. See Central Angle Theorem.