not-a-number

from The Free On-line Dictionary of Computing (8 July 2008)
Not-a-Number
NaN

   <mathematics> (NaN) An {IEEE floating point} representation
   for the result of a numerical operation which cannot return a
   valid number value.  A NaN can result from multiplying an
   infinity by a zero, or from subtracting one infinity from
   another [what else?].

   NaN is encoded as a special {bit pattern} [what pattern?]
   which would otherwise represent a {floating-point} number.  It
   is used to signal error returns where other mechanisms are not
   convenient, e.g. a hardware {floating-point unit} and to allow
   errors to propagate through a calculation.

   Similar bit patterns represent positive and negative
   {overflow} and {underflow} and the positive and negative
   infinities resulting from {division by zero}.

   Bit patterns
   (http://psc.edu/general/software/packages/ieee/ieee.html).

   [ANSI/IEEE Std 754-1985].

   [Correct?]

   (2001-04-01)