The Collaborative International Dictionary of English v.0.48:

HDL \HDL\ ([=a]ch`d[=e]*[e^]l"), n.
   High density lipoprotein, a lipoprotein that transports
   cholesterol in the blood; high levels are thought to be
   associated with decreased risk of coronary heart disease and
   atherosclerosis; sometimes called good cholesterol.
   Contrasted with LDL.

   Syn: high-density lipoprotein.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

HDL
    n 1: a lipoprotein that transports cholesterol in the blood;
         composed of a high proportion of protein and relatively
         little cholesterol; high levels are thought to be
         associated with decreased risk of coronary heart disease
         and atherosclerosis [syn: high-density lipoprotein,
         HDL, alpha-lipoprotein]

The Free On-line Dictionary of Computing (19 January 2023):

Hardware Description Language
HDL

    (HDL) A kind of language used for the conceptual
   design of integrated circuits.  Examples are VHDL and
   Verilog.

   (1995-04-18)