What type of tree traversal processes nodes one at a time?

The most accurate answer to the question is that a tree traversal processes nodes one at a time. "Traversal" is indeed the overarching term that describes the method of visiting nodes in a tree data structure.

Tree traversals are methods to visit and process each node in a tree in a specific order. The common types of tree traversals—such as inorder, preorder, and postorder—each have their own rules for the sequence in which they visit nodes. However, the key concept in all these methods is that they examine or process one node at a time as they visit each part of the tree.

Inorder traversal processes nodes in a left-root-right sequence; preorder processes them in root-left-right order; and postorder does so in left-right-root order. While each of these forms addresses the order of processing, they all fundamentally involve visiting nodes one at a time.

Therefore, the term "traversal" correctly encapsulates the nature of visiting nodes singularly in a systematic way, aligning with the concept asked in the question.