Webster's English Dictionary

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quaternion

qua.ter.ni.on \kw*-'t*r-ne--*n\ n [ME quaternyoun, fr. LL quaternion-, 
   quaternio, fr. L quat]erni four each, fr. quater four times; akin to L 
   quattuor four - more at FOUR 1: a set of four parts, things, or persons : 
   TETRAD  2a: a generalized complex number composed of a real number and a 
   vector and  depending on one real and three imaginary units pl  2b: the 
   calculus of the quaternion