Webster's English Dictionary

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tetrahedron

tet.ra.he.dron \.te-tr*-'he--dr*n\ \-dr*\ n or tetrahedrons or 
   tet.ra.he.dra [NL, fr. LGk tetraedron, neut. of tetraedros having four 
   faces, fr.] pl  Gk tetra- + hedra seat, face - more at SIT : a polyhedron 
   of four faces