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orthogonal: [from mathematics] adj. Mutually independent; well
   separated; sometimes, irrelevant to.  Used in a generalization of
   its mathematical meaning to describe sets of primitives or
   capabilities that, like a vector basis in geometry, span the
   entire `capability space' of the system and are in some sense
   non-overlapping or mutually independent.  For example, in
   architectures such as the PDP-11 or VAX where all or nearly all
   registers can be used interchangeably in any role with respect to
   any instruction, the register set is said to be orthogonal.  Or, in
   logic, the set of operators `not' and `or' is orthogonal,
   but the set `nand', `or', and `not' is not (because any
   one of these can be expressed in terms of the others).  Also used
   in comments on human discourse: "This may be orthogonal to the
   discussion, but...."