Abbreviation: ASL / Asph = "Aspherical" (Tamron)

The abbreviations means "Aspherical"

Explanation from Tamron

Hybrid Aspherical Elements Provide the Ultimate in Image Quality and Compactness

Tamron uses several hybrid aspherical lens elements in many lenses bearing the Aspherical designation. These innovative optics allow us to achieve the ultimate in image quality, and at the same time produce lenses that offer remarkable zoom ranges in extraordinarily compact packages. By perfecting theses cutting-edge advances for series production, Tamron has advanced the state of optical design, and virtually eliminated spherical aberration and image distortion from the high-power-zoom series.Through the effective application of Hybrid Aspherical Technology, one lens elemen tcan take the place of multiple elements without compromising performance. This is what allows us to produce remarkably compact long-range lenses that deliver a uniformly high level of image quality at all focal lengths and apertures.

See examples of effected products

Tamron SP AF 17-50mm f/2,8 XR Di-II LD Asph IF
Tamron AF 28-200mm f/3,8-5,6 XR Di Asph IF Macro

See links

tamron-usa.com