The name "peak" is also occasionally found, in the sense of "top, summit", though this also occurs as a spelling error for the verb "peek".
Behavior when the collection is empty varies – most often this yields an underflow error, identically to a pop on an empty collection, but some implementations provide a function which instead simply returns (without error), essentially implementing if isempty then return, else peek.
For the stack, priority queue, deque, and DEPQ types, peek can be implemented in terms of pop and push (if done at same end).
For priority queues and DEPQs, however, dequeuing and enqueuing often take O(log n) time (for example if implemented as a binary heap), while O(1) performance of "peek" (here generally called "find-min" or "find-max") is a key desired characteristic of priority queues, and thus peek is almost invariably implemented separately.
Making find-min or find-max take O(1) time can be done by caching the min or max values, but this adds overhead to the data structure and to the operations of adding or removing elements.