<jargon> Too small to be worth considering; more specifically, small beyond the limits of accuracy or measurement. This is a reference to "floating point underflow". The Hacker's Jargon File claimed that it is also a pun on "undertow" (a kind of fast, cold current that sometimes runs just offshore and can be dangerous to swimmers). "Well, sure, photon pressure from the stadium lights alters the path of a thrown baseball, but that effect gets lost in the underflow". Compare epsilon, epsilon squared; see also overflow bit. (1997-09-05)

The term arithmetic underflow (also floating point underflow, or just underflow) is a condition in a computer program where the result of a calculation is a number of more precise absolute value than the computer can actually represent in memory on its central processing unit (CPU).

<programming> (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)