The most common meaning for ADSL is "Asymmetric Digital Subscriber Line." It is referring to a form of Digital Subscriber Line, which is a data communications technology.

An asymmetric digital subscriber line is a communications system characterized by moderately high speeds, different in each direction.

Asymmetric Digital Subscriber Line(ADSL)

Copper Telephone Line

ADSL2 stands for 'Asymmetric Digital Subscriber Line 2', and therefore in this case 'A' stands for the word 'Asymmetric', which means to be imbalance and to ignore symmetry, focusing on spatial arrangement