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1. a number of aircraft circling an airport at different altitudes, awaiting their signal to land
2. Brit a measure of coal or wood equal to 108 cubic feet
3. a high column of rock, esp one isolated from the mainland by the erosive action of the sea
4. an area in a computer memory for temporary storage



a rounded mass of compactly piled hay or straw (sometimes in sheaves). A stack is 5–7 m high and has a base circumference of 12–20 m.


(building construction)
The portion of a chimney rising above the roof.
(chemical engineering)
In gas works, a row of benches containing retorts.
(computer science)
A portion of a computer memory used to temporarily hold information, organized as a linear list for which all insertions and deletions, and usually all accesses, are made at one end of the list.
To stand and rack drill rods in a drill tripod or derrick.
Any structure or part thereof that contains a flue or flues for the discharge of gases.
One or more filter cartridges mounted on a single column.
Tall, vertical conduit (such as smokestack, flue) for venting of combustion or evaporation products or gaseous process wastes.
The exhaust pipe of an internal combustion engine.
An erosional, coastal landform that is a steep-sided, pillarlike rocky island or mass that has been detached by wave action from a shore made up of cliffs; applies particularly to a stack that is columnar in structure and has horizontal stratifications. Also known as marine stack; rank.
The cone-shaped section of a blast furnace or cupola above the hearth and melting zone and extending to the throat.
To assign different altitudes by radio to aircraft awaiting their turns to land at an airport.


stack, 1: installation
1. Any vertical pipe, such as a soil pipe, waste pipe, vent, or leader stack.
2. Such pipes, collectively.
3. Any structure or part thereof


A number of aircraft holding over a point but at different flight levels. An aircraft may be asked to hold in a stack.


(See below for synonyms) A data structure for storing items which are to be accessed in last-in first-out order.

The operations on a stack are to create a new stack, to "push" a new item onto the top of a stack and to "pop" the top item off. Error conditions are raised by attempts to pop an empty stack or to push an item onto a stack which has no room for further items (because of its implementation).

Most processors include support for stacks in their instruction set architectures. Perhaps the most common use of stacks is to store subroutine arguments and return addresses. This is usually supported at the machine code level either directly by "jump to subroutine" and "return from subroutine" instructions or by auto-increment and auto-decrement addressing modes, or both. These allow a contiguous area of memory to be set aside for use as a stack and use either a special-purpose register or a general purpose register, chosen by the user, as a stack pointer.

The use of a stack allows subroutines to be recursive since each call can have its own calling context, represented by a stack frame or activation record. There are many other uses. The programming language Forth uses a data stack in place of variables when possible.

Although a stack may be considered an object by users, implementations of the object and its access details differ. For example, a stack may be either ascending (top of stack is at highest address) or descending. It may also be "full" (the stack pointer points at the top of stack) or "empty" (the stack pointer points just past the top of stack, where the next element would be pushed). The full/empty terminology is used in the Acorn Risc Machine and possibly elsewhere.

In a list-based or functional language, a stack might be implemented as a linked list where a new stack is an empty list, push adds a new element to the head of the list and pop splits the list into its head (the popped element) and tail (the stack in its modified form).

At MIT, pdl used to be a more common synonym for stack, and this may still be true. Knuth ("The Art of Computer Programming", second edition, vol. 1, p. 236) says:

Many people who realised the importance of stacks and queues independently have given other names to these structures: stacks have been called push-down lists, reversion storages, cellars, dumps, nesting stores, piles, last-in first-out ("LIFO") lists, and even yo-yo lists!


(1) In a network, a hierarchy of software layers in both clients and servers that are required in order to communicate with each other. See protocol stack.

(2) A hierarchy of software. A stack is the common set of programs used in a computer. The stack can refer to infrastructure only (see technology stack) or to the applications the company gives its employees (see application stack). It may also refer to an industry set or developer set of programs. For example, the phrase "they don't offer a complete stack" could imply that a software company has an incomplete set of applications for a particular industry or niche.

(3) A set of hardware registers or a reserved amount of memory used for arithmetic calculations, local variables or to keep track of internal operations (the sequence of routines called in a program). For example, one routine calls another, which calls another and so on. As each routine is completed, the computer returns control to the calling routine all the way back to the first one that started the sequence. Stacks used in this way are LIFO based: the last item, or address, placed (pushed) onto the stack is the first item removed (popped) from the stack.

Stacks are also used to hold interrupts until they can be serviced. Used in this manner, they are FIFO stacks, in which the first item onto the stack is the first one out of the stack. See internal stack failure, stack dump and stack fault.

(4) A Macintosh folder view (see Stacks).

(5) An earlier Macintosh development system (see HyperCard).