Cluster

That is one possible way of notating clusters, but probably not the most common. The more common symbol in my experience is an elongated notehead. Press "Z" to display the Symbols palette, type "cluster" into the search box, and you'll see various options. If you see one you'd like to use often enough to make it worth adding to the regular palette window, see the Handbook under "Custom palettes" to learn how. if you prefer using the brackets, those are availbale as well, and the same comment about customizing your palette applies.