Threeside

[...] let this base-point be indicated by Ax, while the sixth base-point will be indicated by C. Now [c*] contains a threeside formed by a, [...]

Pappus tells us that Menelaus in his Sphaertca calls the figure in question (the spherical triangle) a 'threeside' [...]

A triangle is then defined as the figure formed by the aggregate of all the segments joining the respective vertices of a three-side to points on the opposite sides.

If a straight line passes through a point of one of the sides of a threeside it must contain at least one point of one of the other sides...For the fourside we define the area to be the sum of the areas of the four threesides in which it is divided by its diagonals.