A queue is an ordered list in which all inserts take place at one end, the rear, while all deletion take place at the other end, the front. This definition matches the concept of a queue, where the "First-In-First-Out" (FIFO) principle is applied. Therefore, the most suitable word to fill in the blank is "queue." A stack, on the other hand, follows the "Last-In-First-Out" (LIFO) principle, which does not match the given description. A graph and a tree are data structures that do not fit the description of ordered insertion and deletion at opposite ends.