Radius
a segment with one end point of the center and one end point on the circle
Diameter
a straight line that passes through the center and whose endpoints lie on the circle
Chord
a segment whose endpoints are on the circle
secant
a line/ray that intersects a circle at 2 points
Tangent
a line/ray/segment that intersects the circle at exactly one point
Arc
a part of a circle
minor arc
less that 180 degrees of a circle
major arc
more than 180 degrees of a circle
semicircle
180 degrees of a circle
concentric
circles/arc/other shapes that share the same center
central angle
an angle with its vertex at the center of the circle
inscribed angle
an angle whose vertex is on the circle
inscribed polygon
a polygon placed inside a circle so that all the vertices lie on the circle
circumscribed polygon
a polygon whose vertices are outside the circle and the sides are tangent to the circle