Abbreviation: One Touch FC = "One Touch Focus Clutch Mechanism" (Tokina)

The abbreviations means "One Touch Focus Clutch Mechanism"

Explanation from Tokina

A refinement of the focus clutch mechanism has been incorporated in the AT-X 280 AF PRO Nikon and Canon mount, so that now the lensí focus ring can be snapped back to the manual focus position at any time, from any position. This improvement means it is not necessary to change the AF/MF switch (on the body with Nikon or the lens with Canon) every time the lens is changed from auto focus to manual focus mode.

See examples of effected products

Tokina AT-X 16-28mm f/2,8 PRO FX
Tokina AT-X Pro 11-16mm f/2,8 DX
Tokina AT-X Pro 35/2,8 DX Macro

See links

tokinalens.com