Abbreviation: IF = "Internal Focusing System" (Tamron)

The abbreviations means "Internal Focusing System"

Explanation from Tamron

Internal focusing (IF) provides numerous practical benefits to photographers including a non-rotating front filter ring that facilitates the positioning of polarizing and graduated filters, and more predictable handling because the lens length does not change during focusing. Even more important, Tamronís Internal Focusing (IF) system provides a much closer minimum focusing distance (MFD) throughout its entire focusing range. In addition, IF improves optical performance by minimizing illumination loss at the corners of the image field, and helps to suppress other aberrations that become more troublesome at different focusing positions.

See examples of effected products

Tamron SP AF 17-50mm f/2,8 XR Di-II LD Asph IF
Tamron SP 200-500mm f/5-6,3 Di LD IF
Tamron SP AF 180mm f/3,5 Di LD IF Macro

See links

tamron-usa.com