A Right Triangle
Right triangles have unique characteristics that classify them as right triangles.
 
Right triangles, like all triangles, have three sides.  Each side has a special name.  One side is called the hypotenuse.  Both of the other two sides are called legs.  
 
Right triangles also have three angles.  One of the three angles in a right triangle is a right angle.  The other two angles are acute angles.