Abbreviation: SP = "Super Performance" (Tamron)

The abbreviations means "Super Performance"

Explanation from Tamron

Super Performance for Discriminating Shooters
Tamron SP (Super Performance) series is a line of ultra-high-performance lenses designed and manufactured to the exacting specifications demanded by professionals and others who require the highest possible image quality. In creating SP lenses Tamron’s optical designers put their foremost priority on achieving superior performance parameters—they are all designed to a higher standard with little regard for cost constraints. As a result, Tamron lenses bearing the SP designation feature impressive and innovative designs that have established an enviable reputation for excellence among those knowledgeable photographers that demand the very best.

See examples of effected products

Tamron SP AF 90mm f/2,8 Di Macro
Tamron SP 11-18mm f/4,5-5,6 Di-II
Tamron SP AF 17-50mm f/2,8 XR Di-II LD Asph IF

See links

tamron-usa.com