Source : Free On-Line Dictionary of Computing

Not-a-Number
     
         (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://www.psc.edu/general/software/packages/ieee/ieee.html)}.
     
        [ANSI/IEEE Std 754-1985].
     
        [Correct?]
     
        (2001-04-01)