The Sphere of the World (De Sphaera)

When a straight line meets a straight line and creates two equal angles on either side, each of them is said to be perpendicular to the other, and each of those angles is denominated a right angle.
1 Item, whatever angle is similar and equal to one of these, it will be a right angle. Wherefore, all rectilinear angles are equated to a right angle.
2 If, however, a straight line meeting a straight line causes two unequal angles on its sides, each of those lines is said to be oblique or inclined upon the other; and the greater angle is called obtuse, and the smaller, acute; wherefore every rectilinear angle greater than a right angle is obtuse, and every one smaller than a right is acute.
A body is that which contains length, breadth, and depth. A figure is that which is enclosed by a boundary or boundaries. Nothing is figured except a surface or a body.
[ornament] Two straight lines do not enclose a surface; therefore, they do not form a figure.
Two straight surfaces together do not define a body nor a figure.
If there are two straight lines or surfaces and they are produced on either side so that they never meet, they are called equidistant. The equidistance of two things is when all the lines stretched most directly from one to the other have been equal.
ly A circle is a flat figure contained by one line which is called the circumference or periphery, in whose middle is a point which is called the center of the circle, from which all straight lines drawn to the circumference are equal.
2 A straight line, however, which passes through the center and applies its extremities to the circumference, divides the circle into two halves, and it is called the diameter. And each of those parts of the circle is called a semicircle and a portion of the circle.
But if a straight line should cut the circle into two unequal parts, it is called a chord, and the larger part of the circle is the major portion, and the smaller part of the circle is the minor portion; and any part of the circumference whatsoever is called an arc; therefore, a portion of a circle is formed from an arc and a chord.

Diagram of a circle divided by a horizontal diameter line. The upper and lower halves are labeled: "Semicircle", "Diameter", "Semicircle."

Boxed diagram showing a vertical line meeting a horizontal line at a ninety-degree angle, labeled "perpendicular."

Boxed diagram showing a line segment meeting a horizontal line at an obtuse angle, labeled "oblique."

Boxed diagram illustrating multiple types of angles sharing a single vertex on a horizontal baseline, labeled "acute," "angle," "right," "obtuse."

Boxed diagram showing two perfectly horizontal parallel lines, labeled "parallel lines."

Boxed diagram showing two lines that converge and intersect toward the right, labeled "intersecting lines."

Geometric diagram of a circle with a horizontal chord cutting off a smaller segment at the top, labeled "minor portion," "chord," "major portion."