What is the meaning of 'breadth' in breadth first search?
Consider the data structure used to represent the search. In a BFS, you use a queue. If you come across an unseen node, you add it to the queue. 

The “frontier” is the set of all nodes in the search data structure. The queue will will iterate through all nodes on the frontier sequentially, thus iterating across the breadth of the frontier. DFS will always pop the most recently discovered state off of the stack, thus always iterating over the deepest part of the frontier. 

Consider the image below. Notice how the DFS goes straight to the deepest parts of the tree whereas BFS iterates over the breadth of each level.



Image here