The story
From amplitude + -hedron. Coined by Nima Arkani-Hamed and Jaroslav Trnka in 2013 and published in 2014 in an article in the Journal of High Energy Physics.
Etymology from Wiktionary, CC BY-SA 4.0.
amplituhedron
Today it means: A mathematical structure which encodes the integrand of an amplitude in planar N=4 super Yang-Mills theory, used in calculations of scattering amplitudes of particle interactions. Full definition →