What is the name of a 3D triangle?

Answer 1

Strictly speaking, if it's a proper 3D triangle, it's a tetrahedron. It has four sides, all triangular. If it has four triangular sides and a square base, it's a pyramid.

Someone else suggested that a triangle taken into the 3rd dimension would be a prism or pentahedron - which is not correct.

A polyhedron is any 3D enclosed shape. These are then named according to the number of faces they have. So a 3D triangle where all faces have 3 sides has a total of 4 faces. ie. tetrahedron. A pyramid, where 4 faces have 3 sides and the bottom has 4 sides has a total of 5 faces, ie. pentrahedron.

Neither a pyramid nor a tetrahedron are prisms. A prism has two opposite faces with exactly the same shape, and these two faces are joined by parallelograms. eg. the two opposite sides are triangles (one at the top, one at the bottom) and they're joined by 3 rectangles. (In that case, you have 5 faces, so it *is* a pentahedron, but not a pyramid. "Pyramid" is a more specific term than "pentahedron".) A prism could also be made with a square on each side, joined by 4 rectangles - it would then have 6 sides, which is also called a hexahedron. The ~hedron names for shapes thus ONLY give the number of faces, not how these are arranged. In the case of the tetrahedron, though, there's only one possible arrangement: A 3-DIMENSIONAL TRIANGLE. There are some helpful pictures on the Student Britannica: http://student.britannica.com/eb/art/print?id=53378&articleTypeId=0