The semi-diameter of a heavenly body is half the angle Subtended by the diameter of the visible disk at the eye of the observer.
"Lectures in Navigation"
Ernest Gallaudet Draper
The angle thus measured is called "the angle Subtended" by the window at the place where the measurement was made.
"A Text-Book of Astronomy"
George C. Comstock
The angular distance of these two stars in the heavens is not so great as the angle Subtended by a line an inch long viewed at a distance of half a mile.
"The Story of the Heavens"
Robert Stawell Ball