splitbutton
nounUK /ˈsplɪtˌbʌtən/
US /ˈsplɪtˌbʌtən/
A split button is a button with two parts that you can click to do different things.
Click the arrow on the split button to see more choices.
OTHER MEANINGS
2. A button in a user interface that combines a default action with a dropdown menu of alternatives.
The split button lets you save quickly or pick Save As from its menu.
3. A physical control divided into two separately pressable sections.
The camera's split button fires the shutter on one side and focuses on the other.
4. A toggle that performs one action on a short press and another on a long press.
The split button on the remote switches channels with a tap and opens settings with a hold.