1.  triangle(n = noun.shape) trigon, trilateral - a three-sided polygon;
is a kind of polygon, polygonal shape
has particulars: acute-angled triangle, acute triangle, equiangular triangle, equilateral triangle, isosceles triangle, oblique triangle, obtuse-angled triangle, obtuse triangle, right-angled triangle, right triangle, scalene triangle, cuneus, wedge, wedge shape
Derived forms adjective triangular₂, adjective triangular₁, verb triangulate₃, verb triangulate₁

2.  triangle(n = noun.shape) - something approximating the shape of a triangle; "the coastline of Chile and Argentina and Brazil forms two legs of a triangle"
is a kind of form, shape

3.  triangle(n = noun.object) triangulum - a small northern constellation near Perseus between Andromeda and Aries;
is a kind of constellation

4.  triangle(n = noun.artifact) - any of various triangular drafting instruments used to draw straight lines at specified angles;
is a kind of drafting instrument

5.  triangle(n = noun.artifact) - a percussion instrument consisting of a metal bar bent in the shape of an open triangle;
is a kind of percussion instrument, percussive instrument

1.  A figure bounded by three lines, and containing three angles. [1913 Webster]
" A triangle is either plane, spherical, or curvilinear, according as its sides are straight lines, or arcs of great circles of a sphere, or any curved lines whatever. A plane triangle is designated as scalene, isosceles, or equilateral, according as it has no two sides equal, two sides equal, or all sides equal; and also as right-angled, or oblique-angled, according as it has one right angle, or none; and oblique-angled triangle is either acute-angled, or obtuse-angled, according as all the angles are acute, or one of them obtuse. The terms scalene, isosceles, equilateral, right-angled, acute-angled, and obtuse-angled, are applied to spherical triangles in the same sense as to plane triangles." [1913 Webster]

2.  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. [1913 Webster]

3.  A draughtsman's square in the form of a right-angled triangle. [1913 Webster]

4.  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. [1913 Webster]

5.  A small constellation situated between Aries and Andromeda. [1913 Webster]

triangle, n.
1 a plane figure with three sides and angles.
2 any three things not in a straight line, with imaginary lines joining them.
3 an implement of this shape.
4 a musical instrument consisting of a steel rod bent into a triangle and sounded by striking it with a small steel rod.
5 a situation, esp. an emotional relationship, involving three people.
6 a right-angled triangle of wood etc. as a drawing-implement.
7 Naut. a device of three spars for raising weights.
8 hist. a frame of three halberds joined at the top to which a soldier was bound for flogging.

Idiom:
triangle of forces a triangle whose sides represent in magnitude and direction three forces in equilibrium.

Etymology:
ME f. OF triangle or L triangulum neut. of triangulus three-cornered (as TRI-, ANGLE(1))

N  triality, trinity, triunity, three, triad, triplet, trey, trio, ternion, leash, shamrock, tierce, spike-team, trefoil, triangle, trident, triennium, trigon, trinomial, trionym, triplopia, tripod, trireme, triseme, triskele, triskelion, trisula, third power, cube, cube root, three, triform, trinal, trinomial, tertiary, ternary, triune, triarch, triadie, triple, tri-, tris-, tria juncta in uno.

N  scourge, rod, cane, stick, ratan, rattan, birch, birch rod, azote, blacksnake, bullwhack, chicote, kurbash, quirt, rawhide, sjambok, rod in pickle, switch, ferule, cudgel, truncheon, whip, bullwhip, lash, strap, thong, cowhide, knout, cat, cat o'nine tails, rope's end, pillory, stocks, whipping post, cucking stool, ducking stool, brank, trebuchet, trebuket, triangle, wooden horse, iron maiden, thumbscrew, boot, rack, wheel, iron heel, chinese water torture, treadmill, crank, galleys, scaffold, block, ax, guillotine, stake, cross, gallows, gibbet, tree, drop, noose, rope, halter, bowstring, death chair, electric chair, gas chamber, lethal injection, firing squad, mecate, house of correction, goaler, jailer, executioner, electrocutioner, lyncher, hangman, headsman, Jack Ketch.

N  angularity, angularness, aduncity, angle, cusp, bend, fold, notch, fork, bifurcation, elbow, knee, knuckle, ankle, groin, crotch, crutch, crane, fluke, scythe, sickle, zigzag, kimbo, akimbo, corner, nook, recess, niche, oriel, coign, right angle, obliquity, angle of 45 degrees, miter, acute angle, obtuse angle, salient angle, reentering angle, spherical angle, angular measurement, angular elevation, angular distance, angular velocity, trigonometry, goniometry, altimetry, clinometer, graphometer, goniometer, theodolite, sextant, quadrant, dichotomy, triangle, trigon, wedge, rectangle, square, lozenge, diamond, rhomb, rhombus, quadrangle, quadrilateral, parallelogram, quadrature, polygon, pentagon, hexagon, heptagon, octagon, oxygon, decagon, pyramid, cone, Platonic bodies, cube, rhomboid, tetrahedron, pentahedron, hexahedron, octahedron, dodecahedron, icosahedron, eicosahedron, prism, pyramid, parallelopiped, curb roof, gambrel roof, mansard roof, angular, bent, crooked, aduncous, uncinated, aquiline, jagged, serrated, falciform, falcated, furcated, forked, bifurcate, zigzag, furcular, hooked, dovetailed, knock kneed, crinkled, akimbo, kimbo, geniculated, oblique, fusiform, wedge-shaped, cuneiform, cuneate, multangular, oxygonal, triangular, trigonal, trilateral, quadrangular, quadrilateral, foursquare, rectangular, square, multilateral, polygonal, cubical, rhomboid, rhomboidal, pyramidal.

N  musical instruments, band, string-band, brass-band, orchestra, orchestrina, monochord, polychord, harp, lyre, lute, archlute, mandola, mandolin, mandoline, guitar, zither, cither, cithern, gittern, rebeck, bandurria, bandura, banjo, bina, vina, xanorphica, viol, violin, fiddle, kit, viola, viola d'amore, viola di gamba, tenor, cremona, violoncello, bass, bass viol, base viol, theorbo, double base, contrabasso, violone, psaltery, bow, fiddlestick, piano, pianoforte, harpsichord, clavichord, clarichord, manichord, clavier, spinet, virginals, dulcimer, hurdy-gurdy, vielle, pianino, Eolian harp, organ, harmonium, harmoniphon, American organ, barrel organ, hand organ, accordion, seraphina, concertina, humming top, flute, fife, piccolo, flageolet, clarinet, claronet, basset horn, corno di bassetto, oboe, hautboy, cor Anglais, corno Inglese, bassoon, double bassoon, contrafagotto, serpent, bass clarinet, bagpipes, union pipes, musette, ocarina, Pandean pipes, reed instrument, sirene, pipe, pitch-pipe, sourdet, whistle, catcall, doodlesack, harmoniphone, horn, bugle, cornet, cornet-a-pistons, cornopean, clarion, trumpet, trombone, ophicleide, French horn, saxophone, sax, buglehorn, saxhorn, flugelhorn, althorn, helicanhorn, posthorn, sackbut, euphonium, bombardon tuba, cymbal, bell, gong, tambour, tambourine, tamborine, drum, tom-tom, tabor, tabret, tabourine, taborin, side drum, kettle drum, timpani, tympani, tymbal, timbrel, castanet, bones, musical glasses, musical stones, harmonica, sounding-board, rattle, tam-tam, zambomba, reed, tuning fork, triangle, Jew's harp, musical box, harmonicon, xylophone, sordine, sordet, sourdine, sourdet, mute, (iv) PERCEPTION OF SOUND.

acute triangle, acute-angled triangle, bermuda triangle, equiangular triangle, equilateral triangle, isosceles triangle, oblique triangle, obtuse triangle, obtuse-angled triangle, right triangle, right-angled triangle, scalene triangle, southern triangle, spherical triangle