Definition from a geometry textbook:
A triangle is a geometric figure consisting of 3 points and straight lines connecting these points.
A triangle is a figure from plane geometry (on a plane, not in volume); it can be drawn, and you can touch the plane on which it is drawn (if it is material). However, it is an abstraction, and you cannot touch it; it is comparable to a thought or a dream... you understand me... a model can have the shape of a triangle, but it is just a model...