The Collaborative International Dictionary of English v.0.48:

LDL \LDL\ ([e^]l`d[=e]*[e^]l/") n. [acronym from Low-Density
   Lipoprotein.] (Med., Biochem.)
   Low-density lipoprotein, a lipoprotein that transports
   cholesterol in the blood; high levels are thought to be
   associated with increased risk of coronary heart disease and
   atherosclerosis; sometimes called informally bad
   cholesterol.

   Syn: low-density lipoprotein.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

low-density lipoprotein
    n 1: a lipoprotein that transports cholesterol in the blood;
         composed of moderate amount of protein and a large amount
         of cholesterol; high levels are thought to be associated
         with increased risk of coronary heart disease and
         atherosclerosis [syn: low-density lipoprotein, LDL,
         beta-lipoprotein]