Final Answer:
The keyboard shortcut for a transition that applies to both audio and video is "Shift + D" in many video editing software applications.
Step-by-step explanation:
In video editing software, transitions are used to smoothly blend one clip into the next. The keyboard shortcut "Shift + D" is commonly employed to apply a default transition that affects both the audio and video components of the clips. This shortcut is a convenient way to quickly add a transition without having to navigate through menus or use the mouse. It enhances the efficiency of the editing process, making it more streamlined for users who want a transition to be applied to both the audio and video tracks simultaneously.