A split menu button is a button that provides a user with a default action and a drop down menu of additional variations of that action.
Split menu buttons have two separate click areas:
Example:
Example of a split menu button.
I've change this from what was written so far in the wiki
from wiki -- Menu buttons are buttons that open a drop-down menu on click. They differ from normal buttons by a small arrow that is displayed next to the button's text label or icon (toolbar button).
I think this is wrong - you need an additional button to then initiate the action, it doesn't make sense - it's a drop down...I think it's only a split button however, some drop downs look like text fields, and some like buttons (but it's not a button issue, it's a drop down issue, due to definition of button...)