threeside

• three-side

From three +‎ side.

threeside (plural threesides)

1. (rare, nonstandard) Any object or shape consisting of three sides; triangle.
   • 1919, Koninklijke Nederlandse Akademie van Wetenschappen, Proceedings of the section of sciences:

     [...] 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, [...]

   • 1921, Sir Thomas Little Heath, A History of Greek Mathematics:

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

   • 1926, Euclid, translated by Thomas L. Heath, The Thirteen Books of Euclid's Elements, 2nd edition, Cambridge, England: Cambridge University Press, translation of original in Ancient Greek:

     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.

   • 1959, Simon Stevin, (Please provide the book title or journal name):

     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.

threeside (not comparable)

1. (rare, nonstandard) Having three sides; three-sided, triangular.

• etherised