A curvilinear angle has its legs curves.
A right angle is formed by one line perpendicular to another; the measure of which is an arc of 90°.
An acute angle is less than a right angle, or than 90°.
An obtuse angle is greater than a right angle.
An oblique angle may be either acute, or obtuse.
The circumference, or periphery of a circle is the curved line which bounds it, being everywhere equally distant from the centre. The circumference is supposed to be divided into 360 degrees (marked thus °); each degree into 60 minutes, each minute (′) into 60 seconds (″).
An arc is any part of the circumference of a circle.
A chord, or subtense, is a right line joining the extremities of an arc.
The radius of a circle is a right line drawn from the centre to the circumference.
The diameter of a circle is a right line drawn through the centre, and terminated by the circumference.