Source : Free On-Line Dictionary of Computing

underflow
     
         (or "floating point underflow", "floating
        underflow", after "{overflow}") A condition that can occur
        when the result of a {floating-point} operation would be
        smaller in magnitude (closer to zero, either positive or
        negative) than the smallest quantity representable.  Underflow
        is actually (negative) {overflow} of the {exponent} of the
        {floating point} quantity.  For example, an eight-bit {twos
        complement} exponent can represent multipliers of 10^-128 to
        10^127.  A result less than 10^-128 would cause underflow.
     
        Depending on the {processor}, the programming language and the
        {run-time system}, underflow may set a status bit, raise an
        {exception} or generate a {hardware} {interrupt} or some
        combination of these effects.  Alternatively, it may just be
        ignored and zero substituted for the unrepresentable value,
        though this might lead to a later {divide by zero} error which
        cannot be so easily ignored.
     
        (1997-08-25)