triangle

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The plane figure formed by connecting three points not in a straight line by straight line segments; a three-sided polygon.
• noun Something shaped like such a figure.
• noun Any of various flat, three-sided drawing and drafting guides, used especially to draw straight lines at specific angles.
• noun Music A percussion instrument consisting of a piece of metal in the shape of a triangle open at one angle.
• noun A relationship involving three people, especially a ménage à trois.

from The Century Dictionary.

• noun A triangular box-fish, as Sectophrys trigonus, of the family Ostraciidæ of the West Indies.
• noun In angling, an arrangement of three fish-hooks bound together with the points outward, forming a triangle.
• Three-cornered; three-angled; triangular.
• noun 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.
• noun Any three-cornered or three-sided figure, body, or arrangement; anything having a triangular form or bounding a three-sided space.
• noun 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.
• noun [capitalized] In astron, same as Triangulum.
• noun Eccles., a symbol of the Trinity.
• noun A chest made in triangular form to hold a priest's cope.
• noun A three-cornered straight-edge, with one right angle and the other angles more or less acute, used in conjunction with the T-square for drawing parallel, perpendicular, or diagonal lines.
• noun A kind of gin for raising heavy weights, formed by three spars joined at top. See gin, 2 .
• noun 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.
• noun In ceramics, a form of the stilt consisting of three metal pins held together in the form of a triangle. See stilt, 5.
• noun One of certain tortricid moths: an English collectors' name. Tortrix rufana is the red triangle.
• noun In entomology, a large three-sided cell found in the wings of many dragon-flies.
• noun See conjugate triangles, under conjugate.
• noun A triangle whose sides are rectilinear.
• noun the circumscribed circle;
• noun the inscribed and the three escribed circles;
• noun the Feuerback or nine-point circle;
• noun the Brocard or seven-point circle;
• noun the Tucker or triplicate-ratio cirde;
• noun the sine triple-angle 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)
• noun the Taylor or six-point 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
• noun the Spieker circle, or circle inscribed in the triangle whose vertices are the mid-points of the sides of the primitive triangle. See circle.
• noun the centroid, or intersection of median lines
• noun the orthocenter, or intersection of perpendiculars from the angles upon the opposite sides
• noun the circumcenter, or center of the circumscribed circle
• noun the center of the Feuerbach circle;
• noun the incenter, or center of the inscribed circle
• noun the radical center of the escribed circles;
• noun 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
• noun the Spieker point, or mid-point between the circumcenter and incenter
• noun 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')
• noun (10) the center of the triplicate-ratio circle; besides others.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) A figure bounded by three lines, and containing three angles.
• noun (Mus.) 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.
• noun A draughtsman's square in the form of a right-angled triangle.
• noun (Mus.) 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.
• noun A small constellation situated between Aries and Andromeda.
• noun A small constellation near the South Pole, containing three bright stars.
• noun (Zoöl.) a small American spider (Hyptiotes Americanus) of the family Ciniflonidæ, living among the dead branches of evergreen trees. It constructs a triangular web, or net, usually composed of four radii crossed by a double elastic fiber. The spider holds the thread at the apex of the web and stretches it tight, but lets go and springs the net when an insect comes in contact with it.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geometry A polygon with three sides and three angles.