Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. The plane figure formed by connecting three points not in a straight line by straight line segments; a threesided polygon.
 n. Something shaped like such a figure: a triangle of land.
 n. Any of various flat, threesided drawing and drafting guides, used especially to draw straight lines at specific angles.
 n. Music A percussion instrument consisting of a piece of metal in the shape of a triangle open at one angle.
 n. A relationship involving three people, especially a ménage à trois.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A polygon with three sides and three angles.
 n. A percussion instrument made by forming a metal rod into a triangular shape which is open at one angle. It is suspended from a string and hit with a metal bar to make a resonant sound.
 n. A triangular shaped piece of equipment used for gathering the balls into the formation required by the game being played.
 n. A love triangle.
 n. The structure of systems composed with three interrelated objects.
from the GNU version of the Collaborative International Dictionary of English
 n. A figure bounded by three lines, and containing three angles.
 n. An instrument of percussion, usually made of a rod of steel, bent into the form of a triangle, open at one angle, and sounded by being struck with a small metallic rod.
 n. A draughtsman's square in the form of a rightangled triangle.
 n. A kind of frame formed of three poles stuck in the ground and united at the top, to which soldiers were bound when undergoing corporal punishment,  now disused.
 n.
 n. A small constellation situated between Aries and Andromeda.
 n. A small constellation near the South Pole, containing three bright stars.
from The Century Dictionary and Cyclopedia
 Threecornered; threeangled; triangular.
 n. In geometry, a figure composed of three lines which meet two by two in three points, called the vertiees of the triangle; especially, a rectilinear figure of this description.
 n. Any threecornered or threesided figure, body, or arrangement; anything having a triangular form or bounding a threesided space.
 n. A musical instrument of percussion, made of a rod of polished steel bent into the form of a triangle, and open at one of its angles.
 n. [capitalized] In astron, same as Triangulum.
 n. Eccles., a symbol of the Trinity.
 n. A chest made in triangular form to hold a priest's cope.
 n. A threecornered straightedge, with one right angle and the other angles more or less acute, used in conjunction with the Tsquare for drawing parallel, perpendicular, or diagonal lines.
 n. A kind of gin for raising heavy weights, formed by three spars joined at top. See gin, 2 .
 n. Milit., formerly, in the British army, a sort of frame formed of three halberds stuck in the ground and united at the top, to which soldiers were bound to be flogged: generally in the plural.
 n. In ceramics, a form of the stilt consisting of three metal pins held together in the form of a triangle. See stilt, 5.
 n. One of certain tortricid moths: an English collectors' name. Tortrix rufana is the red triangle.
 n. In entomology, a large threesided cell found in the wings of many dragonflies.
 n. See conjugate triangles, under conjugate.
 n. A triangle whose sides are rectilinear.
 n. the circumscribed circle;
 n. the inscribed and the three escribed circles;
 n. the Feuerback or ninepoint circle;
 n. the Brocard or sevenpoint circle;
 n. the Tucker or triplicateratio cirde;
 n. the sine tripleangle circle (constructed as follows: on the sides of the triangle ABC take D and D' on BC, E and E' on AC, F and F' on AB such that the angle AEF =AF'E' =A, BFD =BD'F' =B, CDE =CE'D' =C; then the circle in question passes through D, D', E, E', F, F', and DD': EE': FF'=sin 3A: sin 3B: sin 3C)
 n. the Taylor or sixpoint circle, which passes through the six feet of perpendiculars drawn to the sides from feet of perpendiculars on the sides from the vertices of the triangle
 n. the Spieker circle, or circle inscribed in the triangle whose vertices are the midpoints of the sides of the primitive triangle. See circle.
 n. the centroid, or intersection of median lines
 n. the orthocenter, or intersection of perpendiculars from the angles upon the opposite sides
 n. the circumcenter, or center of the circumscribed circle
 n. the center of the Feuerbach circle;
 n. the incenter, or center of the inscribed circle
 n. the radical center of the escribed circles;
 n. the symmedian, Grebe, or Lemoine point, the intersection of the three lines each bisecting a side and bisecting a perpendicular from an angle upon a side
 n. the Spieker point, or midpoint between the circumcenter and incenter
 n. the Brocard points, two points of the Brocard circle (which see, under circle) (through the symmedian point S of any triangle ABC lines are drawn parallel to the sides of the latter, meeting these sides in D and D' on BC, E and E' on AC, F and F' on AB, so that D, S. E' are collinear, as well as E, S, F' and F, S, D'; then the three lines through A parallel to FD, through B parallel to DE, and through C parallel to EF meet in one Brocard point P, while the lines through A parallel to D' E', through B parallel to E'F', and through C parallel to F' D' meet in the other Brocard point P')
 n. (10) the center of the triplicateratio circle; besides others.
 n. A triangular boxfish, as Sectophrys trigonus, of the family Ostraciidæ of the West Indies.
 n. In angling, an arrangement of three fishhooks bound together with the points outward, forming a triangle.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. any of various triangular drafting instruments used to draw straight lines at specified angles
 n. a percussion instrument consisting of a metal bar bent in the shape of an open triangle
 n. something approximating the shape of a triangle
 n. a threesided polygon
 n. a small northern constellation near Perseus between Andromeda and Aries
Etymologies
Examples

After a long series of experiments, guided by his ideas of the genesis of geometric figures, Séguin became aware that the triangle is the figure most easily drawn.

The Moslems, I am told, cut three locks – a triangle is a favourite Moslem tattoo pattern.

Because when I go out on head coaching interviews and if I mention the word 'triangle,' it makes general managers and owners cringe.

County Administrator Will Johnson suggested they only do what they call the triangle, which excludes a portion of the north part of the county.

Locke, that it is as inconceivable that the entity known to us as Matter should possess the property of causing thought as it is that the figure which we term a triangle should posses the property of containing more than two right angles, still it remains, for the purposes of Locke's supposed theistic demonstration, to prove that it is an inconceivable for the entity which we call Mind _not_ to be due to another Mind, as it is for a triangle

The relationship advice on the love triangle is actually a very productive way to counter some of the negative things that many people have to say about the Twilight Saga.

CHECK OUT THE JACOB BARBIE! www. people.com: And the love triangle is complete!
TWILIGHT SAGA NEWS FOR NOVEMBER 16TH  Open Society Book Club Discussions and Reviews

For example, three Woolworths sites around Birmingham form an exact equilateral triangle (Wolverhampton, Lichfield and Birmingham stores) and if the base of the triangle is extended, it forms a 173.8 mile line linking the Conway and Luton stores.

Somewhere in the DoloresGuanajuatoSan Miguel triangle is an another obvious choice.

Which I understand the temptation, the fight will be more entertaining to watch, but the love triangle is also very central to the book.
yarb commented on the word triangle
The first threesyllable word my daughter uttered. Hail, Euclid!
November 29, 2007