The Collaborative International Dictionary of English v.0.48:

Tubercle \Tu"ber*cle\, n. [L. tuberculum, dim. of tuber: cf. F.
   tubercule, OF. also tubercle. See Tuber.]
   [1913 Webster]
   1. A small knoblike prominence or excrescence, whether
      natural or morbid; as, a tubercle on a plant; a tubercle
      on a bone; the tubercles appearing on the body in leprosy.
      [1913 Webster]

   2. (Med.) A small mass or aggregation of morbid matter;
      especially, the deposit which accompanies scrofula or
      phthisis. This is composed of a hard, grayish, or
      yellowish, translucent or opaque matter, which gradually
      softens, and excites suppuration in its vicinity. It is
      most frequently found in the lungs, causing consumption.
      [1913 Webster]

   Tubercle bacillus (Med.), a minute vegetable organism
      (Mycobacterium tuberculosis, formerly Bacillus
      tuberculosis, and also called Koch's bacillus)
      discovered by Koch, a German physician, in the sputum of
      consumptive patients and in tuberculous tissue. It is the
      causative agent of tuberculosis.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

tubercle bacillus
    n 1: cause of tuberculosis [syn: tubercle bacillus,
         Mycobacterium tuberculosis]