Definitions for trackballˈtrækˌbɔl
Random House Webster's College Dictionary
a computer input device for controlling the pointer on a display screen by rotating a ball set inside a case.
Origin of trackball:
an electronic device consisting of a rotatable ball in a housing; used to position the cursor and move images on a computer screen
"a trackball is essentially an upside-down mouse"
A pointing device consisting of a ball housed in a socket