Abbreviation: MC = "Multi-Coating" (Tokina)

The abbreviations means "Multi-Coating"

Explanation from Tokina

Reflections off the surface of lens elements are the enemy to any photographer and to every lens manufacturer. They are reduced or eliminated by bonding multiple layers of a transparent anti-reflection chemical to the surface of the glass. Tokina has developed and perfected a unique coating technique for all of its optics so that they will maintain faithful color reproduction and render clean, sharp images.

See links

tokinalens.com