1. Noun.  A group or set of four people or things..
  2. Noun.  (mathematics) A four-dimensional hypercomplex number that consists of a real dimension and 3 imaginary ones (''i'', ''j'', ''k'') that are each a square root of -1. They are commonly used in vector mathematics and in calculating the rotation of three-dimensional objects..

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This entry was last updated on RefTopia from its source on 3/20/2012.