Euclid's Elements
Table of Contents Book I Book II Book III Book IV Book V Book VI Book VII Book VIII Book IX Book X Book XI Book XII Book XIII

Definitions

Definition 1.
A rectilinear figure is said to be inscribed in a rectilinear figure when the respective angles of the inscribed figure lie on the respective sides of that in which it is inscribed.
Definition 2.
Similarly a figure is said to be circumscribed about a figure when the respective sides of the circumscribed figure pass through the respective angles of that about which it is circumscribed.
Definition 3.
A rectilinear figure is said to be inscribed in a circle when each angle of the inscribed figure lies on the circumference of the circle.
Definition 4.
A rectilinear figure is said to be circumscribed about a circle when each side of the circumscribed figure touches the circumference of the circle.
Definition 5.
Similarly a circle is said to be inscribed in a figure when the circumference of the circle touches each side of the figure in which it is inscribed.
Definition 6.
A circle is said to be circumscribed about a figure when the circumference of the circle passes through each angle of the figure about which it is circumscribed.
Definition 7.
A straight line is said to be fitted into a circle when its ends are on the circumference of the circle.