The Free On-line Dictionary of Computing (30 December 2018):

underflow
floating point underflow
floating 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 2^-128 to
   2^127.  A result less than 2^-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.

   (2006-11-09)