The Terry Thomas sign refers to an increase in the scapholunate space on an AP radiograph of the wrist (or coronal CT). [See image below]
The increased distance indicates scapholunate dissociation.
The sign is named after well-known British comic Terry Thomas (1911 – 1990) who had frontal dental diastema (a large gap between his two front teeth) .
It is also be known as the David Letterman sign for the same reason.