thymus

thy·mus   (thī'məs)    n.   pl. thy·mus·es A small glandular organ that is situated behind the top of the breastbone, consisting mainly of lymphatic tissue and serving as the site of T cell differentiation. The thymus increases gradually in size and activity until puberty, becoming vestigial thereafter. [New Latin, from Greek thumos, warty excrescence, thymus.]

thymus 

Main Entry: thy·mus
Pronunciation: 'thI-m&s
Function: noun
: a glandular structure of largely lymphoid tissue that functions in cell-mediatedimmunity by being the site where T cells develop, that is present in the young of most vertebrates typically in the upper anterior chest or at the base of the neck, that arises from the epithelium ofone or more embryonic branchial clefts, and that tends to disappear or become rudimentary in the adult called also thymus gland

thymus thy·mus (thī'məs)
n. pl. thy·mus·es

1. A lymphoid organ that is located in the superior mediastinum and lower part of the neck and is necessary in early life for the normal development of immunological function.

2. The thymus of a calf or lamb.