V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

HDSL
       High data / bit rate Digital Subscriber Line (BELLCORE, AT&T,
DSL, BIT)


V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

HDSL
       High-level Data Specification Language


The Free On-line Dictionary of Computing (30 December 2018):

High bit-rate Digital Subscriber Line
HDSL

    (HDSL) A form of Digital
   Subscriber Line, providing T1 or E1 connections over two
   or three twisted-pair copper lines, respectively.  Unlike
   most other forms of DSL HDSL is not a typical consumer
   service, it's used mostly to replace traditional T1/E1
   connections, such as connecting PBXes to telco offices.
   The advantage of HDSL over the Alternate Mark Inversion line
   coding scheme traditionally used on T1/E1 lines is that it
   requires about an order of magnitude lower bandwidth to carry
   the same traffic.

   (1998-05-18)