In computer science, sequential access means that a group of elements (e.g. data in a memory array or a disk file or on a tape) is accessed in a predetermined, ordered sequence. Sequential access is sometimes the only way of accessing the data, for example if it is on a tape. It may also be the access method of choice, for example if we simply want to process a sequence of data elements in order.
In data structures, a data structure is said to have sequential access if one can only visit the values it contains in one particular order. The canonical example is the linked list. Indexing into a list which has sequential access requires O(k) time, where k is the index. As a result, many algorithms such as quicksort and binary search degenerate into bad algorithms that are even less efficient than their naïve alternatives; these algorithms are impractical without random access. On the other hand, some algorithms, typically those which don't perform indexing, require only sequential access, such as mergesort, and so face no penalty.