Final answer:
A highly unbalanced binary search tree is referred to as a degenerate or pathological tree, resembling a linked list with inefficient operations.
Step-by-step explanation:
A binary search tree that is highly unbalanced is called a degenerate or pathological tree. In such a tree, each parent node has only one associated child node which means the tree takes the form of a linked list rather than a well-balanced tree structure. This creates a scenario where the operations in the tree, such as insertions, deletions, and searches, have the worst-case complexity of O(n), where n is the number of nodes in the tree, similar to linear search algorithms, making these operations much less efficient than in a balanced binary search tree where operations are generally O(log n).