A collection of items in which only the earliest added item may be accessed. Basic operations are add (to the tail) or enqueue and delete (from the head) or dequeue. Delete returns the item removed. Also known as "first-in, first-out" or FIFO. Formal Definition: It is convenient to define delete or dequeue in terms of remove and a new operation, front. The operations new(), add(v, Q), front(Q), and remove(Q) may be defined with axiomatic semantics as follows.

1. new() returns a queue
2. front(add(v, new())) = v
3. remove(add(v, new())) = new()
4. front(add(v, add(w, Q))) = front(add(w, Q))
5. remove(add(v, add(w, Q))) = add(v, remove(add(w, Q)))