Abbreviation: AF/MF = "Auto-focus/Manual-focus (AF/MF) Switch Mechanism" (Tamron)

The abbreviation AF/MF means "Auto-focus/Manual-focus (AF/MF) Switch Mechanism" and is seen on Tamron products.

Explanation from Tamron

Auto-focus/Manual-focus (AF/MF) Switch Mechanism on the Focus Ring

The ability to switch quickly between auto-focus (AF) and manual-focus (MF) is a great asset in many types of shooting, especially sports and nature photography. This is easily accomplished with Tamronís ingenious AF/MF system. By simply sliding the focus ring back and forth between the two positions you can select either the AF or MF focusing position. In manual-focus mode Tamron lenses provide smooth, precise manual focusing by turning their wide, well-textured focusing rings.

See links

tamron-usa.com