A triangle is a flat shape with three sides and three angles, like a slice of pizza, a yield sign, or the Greek letter delta. If you look carefully, you'll notice triangles all around you!
A triangle is a polygon, which is a closed shape like a square or a hexagon, but a triangle has only three sides. Wedges of pie and some tortilla chips are shaped like triangles. If you invite your crush to a movie and they bring a date, you have yourself a love triangle. Triangle comes from the Latin word triangulus, "three-cornered" or "having three angles," from the roots tri-, "three," and angulus, "angle or corner."