A Triagon

Triagon (NLT /tr'aıegon/; NaT /traıgon/) 

noun 

A practically unachievable yet theoretically probable geometric shape with only three vertices occupying three dimensions used to explain the scientific theory known as Winter's law (from Winter Leibniz). (Not to be confused with Trigon or Triangle; which is two dimensional) 

rare usage: A crucial concept in a subject difficult to be taught. (i.e: Calculus is a triagon in mathematics) 

-New Millennium Dictionary, 26th extended edition, 2100-