Diameter
A diameter is a line segment that passes through the center of a circle and whose endpoints are on the circle, or the length of that segment.
Example 1
This scene in the the movie Harry Potter and the Sorcerer's Stone takes place at Harry's first Quidditch math at Hogwarts. In this aerial view of the Quidditch field, the line that runs across the center of the field marks the diameter of the circle in the middle of the field.
"By eleven o'clock the whole school seemed to be out in the stands around the Quidditch pitch. Many students had binoculars. The seats might be raised in the air, but it was still difficult to see what was going on sometimes." -Harry Potter and the Sorcerer's Stone, Page 184
Example 2
This scene in the the movie Harry Potter and the Deathly Hallows, Part 1 takes place in the Forest of Dean when Hermione is showing Harry the symbol of the Deathly Hallows in her version of The Tales of Beedle the Bard. The line representing the Elder Wand that runs across the middle of the triangle and the circle represents the diameter of the circle.
This scene is not in the book.