X.
When one straight line standing on another straight line, makes the adjacent angles equal to one another, each of these angles is called a right angle, and each of these lines is said to be perpendicular to the other.
XI.
An obtuse angle is an angle greater than a right angle.
XII.
An acute angle is an angle less than a right angle.
XIII.
A term or boundary is the extremity of any thing.
XIV.
A figure is a surface enclosed by all sides by a line or lines.
XV.
A circle is a plane figure, bounded by one continued line, called its circumference or periphery; and having a certain point within it, from which all straight lines drawn to its circumference are equal.
XVI.
This point (from which the equal lines are drawn) is called the centre of the circle.
