Dictionary
Thumb zone
The area of a touchscreen a thumb can comfortably reach during one-handed use.
In more detail
What It Means
The thumb zone is the region of a phone screen a thumb can comfortably reach while holding the phone one-handed, generally the lower half of the screen. Primary actions placed outside it, especially in the top corners, ask for a two-handed grip or a stretch.
See it in context on the homepage: Touch and Mobile.