The Collaborative International Dictionary of English v.0.48:

thymus \thy"mus\ (th[imac]"m[u^]s), a. [NL., fr. Gr. qy`mos.]
   (Anat.)
   Of, pertaining to, or designating, the thymus gland. -- n.
   The thymus gland.
   [1913 Webster]

   Thymus gland, or Thymus body, a ductless gland in the
      throat, or in the neighboring region, of nearly all
      vertebrates. In man and other mammals it is the throat, or
      neck, sweetbread, which lies in the upper part of the
      thorax and lower part of the throat. It is largest in
      fetal and early life, and disappears or becomes
      rudimentary in the adult. The thymus gland functions as
      the site of maturation of T-lymphocytes (T-cells), which
      confer cell-mediated immunity on the host organism; thus,
      removal or malfunciton of the thymus can lead to absence
      of cell-mediated immunity, and a consequent loss of
      resistance to infection.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

thymus gland
    n 1: a ductless glandular organ at the base of the neck that
         produces lymphocytes and aids in producing immunity;
         atrophies with age [syn: thymus gland, thymus]