underflow

Short definition from Wiktionary

Noun

1. 1
   A current flowing below the surface.
2. 2
   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.
3. 3
   The error condition that results from an attempt to retrieve an item from an empty stack.

Verb

1. 1
   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.
2. 2
   To trigger the error condition resulting from attempting to retrieve a value from an empty stack.