Abbreviation: HLD = "High-refraction, Low Dispersion" (Tokina)

The abbreviations means "High-refraction, Low Dispersion"

Explanation from Tokina

Tokina’s wide-angle and standard zoom lenses feature a higher quality of optical glass known as Tokina HLD (High-refraction, Low Dispersion) glass. Having higher refractive index and lower dispersion properties, HLD glass is far less likely than standard optical glass to create lateral chromatic aberration, which is often a problem with conventionally designed wide-angle lenses

See links

tokinalens.com