From AMS Glossary
The holding pattern of aircraft awaiting their turn to approach and land at an airport.
Stacking aircraft one above the other has to be employed when weather conditions interfere with normal VFR direct landing approach to the extent that aircraft arrive at a more rapid rate than they are able to approach and land. Stacked aircraft are usually separated by a fixed altitude, and when the bottom aircraft lands all those in the stack drop down one altitude separation. Newly arrived aircraft take a place at the top of the stack.