What is the meaning of Underflow?

A current flowing below the surface.

A condition in which the value of a computed quantity is smaller than the smallest non-zero value that can be physically stored; usually treated as an error condition.

The error condition that results from an attempt to retrieve an item from an empty stack.

To trigger the condition in which the value of a computed quantity is smaller than the smallest non-zero value that can be physically stored.

To trigger the error condition resulting from attempting to retrieve a value from an empty stack.

Source: wiktionary.org